Hello everyone =) I'm a noob and this is my first guide. I have experienced some champions thanks to the guides I found here, but there is somenthing I'd like to say to all noob: think like a noob.
Most of guide here are for people who know how to play and know quite great number of Champions. So here I am! A fast look to some (not all) champion ^^

P.S. I said I'm a noob, right? I recomend to read the suggested guide!!! This is only my first one here!

This is not a guide to Champions, but a little reminder to help you choose or not (if noob).

Role: Tank, Support
Main lane: Top, Bottom
Main Damage: AP

Passive: Trample
Q: Pulverize W: Headbutt E: Triumphant Roar R: Unbreakable Will

Alistar is pretty slow and his sustain is not much for the carry (as support) but for the minions. You should gank, because you have not really a 1v1 power, so trust your minions, carry and tower.
For the fact you are so slow you'll be shot at least 2 times in towerdive. Keep in mind that you are not really a tank, in support role, and that your main tanky come from "Ubrekable Will"

Take runes that keep you alive (armor and health) and some CD reduction. As mastries take a look to defensive and supportive, you might as well spent some point for the AP damage, but you'll not kill someone.

Alistar need some good positioning and excell in late game, where your build is complete. You'll run out of mana very quickly even after full build.

Take a look here but read at least 3 guides. Alsitar gamestyle depends on you more than other champions. I don't find good the combo Q-W try instead W-Q to maximize the CC potential.

Role: Carry
Main lane: Bottom
Main Damage: AD

Passive: Focus
Q: Frost Shot W: Volley E: Hawkshot R: Enchanted Crystal Arrow

Ashe is a good champion that provide also some utility for the support (free ward and slow) and can farm easily. Her main disadvantage is the health but also the damage output (she has not incrementation by spells).
I found Ashe competitive, but little hard to use because her ultimate (should be use properly) and the positioning's skill required. Ashe also need a good communication, especially when uses Q and then use mana.

Try to build her aggressive! And take a look here to find out something usefull to Ashe

Ashe is a cheap champion, so a good choice!!!

Role: Jungler, Support (sometimes)
Main lane: Jungle
Main Damage: AP

Passive: A Harmless Scarecrow
Q: Terrify W: Bountiful Harvest E: Reap R: Crowstorm

Fiddlesticks main role is jungler, rarely you'll find him as other role, somethimes as support, but don't count on. Is extremly squishy and, even if provide utility, is easly counterable in jungle.
Considered king of gank thank to the R, he give a bad time to other team. His main spells are W and Q for extreme sustain and damage, but also E is important for teamfight or secure a kill. He can kill Dragon alone.

Be carefull and push as hell in the early-mid game, because he take less utility in late, can be killed easily in late so just be carefull and take a Zhonya's Hourglass.

Take a look here but read at least 3 guides. Maybe AP build is a good one just to start.

Role: Support
Main lane: Bottom
Main Damage: AP

Passive: Tailwind
Q: Howling Gale W: Zephyr E: Eye of the Storm R: Monsoon

You'll hear that Janna is OP, i think she is not. She provide some utility and a nice passive, but the wrong use can really ruin the game.
First of all keep in mind to stay close enough to your Carry and activate your E, ready to be used. For example, I move and the activate ever my E (not using, but showing range) ready for a gank or something else.
If you plan to use your Q, be carefull to the delay time between the cast and the effect. You should predict enemy movement in order to land a good shot. You can also use Q before warding a bush if you fear an enemy and relase is just when you are warding.

Your R is your weak/strong point. You'll be insult because you didn't heal, even if you saved your team. You can use this spell both defensive or offensive, i prefer to use it to split enemy team, along with Q, to disengage and create a sort of 5v3(or even 2) to take the upper hand.

Be carefull to CC, you are fast but squishy and vulnerable to stun & company. I don't see so many Janna AP Mage, but maybe some AP will help to improve your potential. Take a look to Rylai's Crystal Scepter for AP and health.

Tips and trick for Janna goes here =P

Role: Mage-Pusher, Jungler, Support
Main lane: Top-Mid, Jungle, Bottom
Main Damage: AP

Passive: Holy Fervor
Q: Reckoning W: Divine Blessing E: Righteous Fury R: Intervention

Kayle is strong in early level game (less than 20), but is easily counterable when enemy become more expert and know a little bit of everything (basic engage rules, etc). That cheap champions found her full potential in late game.

Born as support is much more stronger as AP and you'll discover that her supportive potential is not that great, but W can make you faster for a little, usefull to go in position or regroup.

Kayle's R is incredible strong, but if used at wrong moment can just ruin a good fight. Don't use it "hoping" to kill the enemy, just play safe and use it at right time (and on right teammate, maybe not you).

There is not much more to say... take a look here to know Kayle

Role: Tank, Support
Main lane: Top, Bottom
Main Damage: AP

Passive: Sunlight
Q: Shield of Daybreak W: Eclipse E: Zenith Blade R: Solar Flare

Ahhh, Leona... my dear sweet Leona... Just kidding xD She is one of my favourite champion. I like to play her much more as Support because the potential of her passive. Build her tanky but also with some magic penetration to help secure a kill for the carry.

Leona seems an easy champion: CC, gap closer, shield... but is more complicated. Leona should be one of most carefull support. Others Tank have little more life, and her supportive role means she can't have so much.

The gold rule: "Engage only if you have situation under controll"

Another thing i don't see so much in Leona's players is the patience to wait. Lots of people spam their spells at once and find themselve Kill Stealing (really easy with Leona) or dying.
Don't waste your passive! Take the enemy with E and wait a little before using Q, that will deal bonus damage (if the carry is awake) and make the target down for much more time (even E "stun" a little).

Pick her when you know well the base engage rules and practice a lot even versus bot. Here you can find out more information about Leona

Leona need a lot of team communication because everytime is in first line.

Role: Support
Main Lane: Bottom
Main Damage: AP

Passive: Pix, Faerie Companion
Q: Glitterlance W: Whimsy E: Help, Pix! R: Wild Growth

Lulu is what we might call a full support. She have: poke, slow and bushes check (Q); interruption and speed/AP buff (W); shield, vision and increase of range (E); initiation and lifesaving (R). Lulu has an extremely varied kit.

Play Champions is much more difficult as many things you need to know and keep an eye. Lulu is pretty that kind of champion.
Her Q can easly check bush, or deal damage, but the way you cast is pretty unique. Two shots goes on two different line (depends on where Pix is), giving you the possibility to perform a 90° degrees shot! Sound awesome? Is not, because the first game you will maybe use both projectile against a single target (you can deal damage only one). This is an example, because all Lulu's spells are pretty useless if used wrong and maybe will bring you to death.

However you can use your E or W on an enemy (or ally), Pix will follow the target and will also shot one projectile of your Q. So, if an enemy retreats often, leaving the companions behind, you can slow all enemy team, dealing also some nice damage.

Personally I do not like her as AP Carry, but you can try. The only real tip is: try a lot and know very well your carry. An error can cost you very much, so read as many guides you can. This one is my favourite for Lulu

With Lulu you can learn how to improve the timing of actions and the use of certain spells.

Role: Jungle, Tank, Mage
Main lane: Jungle, Top
Main Damage: AP

Passive: Visionary
Q: Consume W: Blood Boil E: Ice Blast R: Absolute Zero

Nunu is a cheap champion, pretty versatile. Suitable in any role from Top to Jungle, he have also some utility and nice sustain. He can do well his work and, even if not the stronger jungler, is a nice pick and a nice start.
Nunu is quite simple at first sight, just E and Q. Wrong! (I'm sorry =P) Nunu is not easy because his R. He also should jungle with attention and never forget to eat with Q (because of the bonus) in late game. Quite mana dependent and long CD, if possible wait for the free spell after the five hit granted by the passive to restore a little of mana.

If you play safe enough and build correctly, with mastries and runes, you'll carry some nice game (not really carry to kill if not AP).

I built it by myself because not so satisfied from classic build, take a look to more than 3 and practice. Nunu is item and team dependent.

Role: Mage, Support (pretty never)
Main lane: Mid
Main Damage: AP

Passive: Clockwork Windup
Q: Command: Attack W: Command: Dissonance E: Command: Protect R: Command: Shockwave

Sometimes I heard is OP, sometimes I think there aren't enough stone. Orianna is hard to play, unique style and unique moves.
Orianna is different from any champion, can farm easily but only if you know how to farm. Can zone easily but only if you know how to zone. She Have nice harras potential, but only if you know how to harras. Orianna's story is all like that.

There is not much to say except: be carefull and practice a lot! Read as many guides you can. Build her AP and find good combos with her spells. Simply not simply to use.

Give to Orianna more than a try, but think with your mind.

Your ball will be "stolen" by your teammates, so try to communicate when you need it.

Role: Carry
Main lane: Bottom
Main Damage: AD

Passive: Fleet of Foot
Q: Boomerang Blade W: Ricochet E: Spell Shield R: On The Hunt

Better known as "ARGH! My eyes!!!" because her incredible low resolution model, she is (my opinion) a great ADC. Incredibly cheap, do not be fooled by the price, if you are not able to use her E you just waste half of her utility. Need to know enemy's spells.
Incredibly mana hungry when use her Q, she require a little positioning but if you are at level 20 or more you should be fine. Build her AD, you'll kill a wave of minion simply W and Q only one time. Sivir is maybe the best pusher and is able to zone the enemy, keeping the distance.

Sivir builds are usually outdated, but all you need is here.

Role: Support
Main lane: Bottom
Main Damage: AP

Passive: consecration
Q: Starcall W: Astral Blessing E: Infuse R: Wish

Cheap champion, the best to start as support. Soraka have not many aggressive potential so she is the right one to start as support and learn that role.
She can look quite easy, but in-depth strategy and team communication can make her a good and solid choice. Soraka (as Alistar) can also minions support, making her a great Aura champion (even her passive is an aura) and a great healer (R is an incredible global heal).

The only bad things are CD time, really long. If need more about Soraka try the guide.

Role: Jungler, Tank
Main lane: Jungle, Top (rare)
Main Damage: Depends, mainly AD

Passive: Bridge Between
Q: Wilding Claw W: Iron Mantle E: Blazing Stampede R: Wingborne Storm

Udyr is a great jungler, fast, aggressive and with nice survability. Can counter easily thanks to all the stance, nice gank but no performing well in lane (is only melee). This is the only downgrade, in fact he is able to destroy towers really quickly.
Read as many guide as you can about Jungling, about Udyr and how to counter-jungle. This champion is not easy because the stance offers different way to play and is only strict for close combat; however can kill Dragon alone.

Role: Mage
Main lane: Mid
Main Damage: AP

Passive: ascended form
Q: Arcanopulse W: locus of power E: mage chains R: Arcane Barrage

Xerath, my love! Ok, sorry xD Xerath is a skillshot champion, a mage. His true power is in mid-late when all spells are unlocked. Cooldown is not a problem on Xerath, even the R is pretty fast and can be used 3 times before CD (you have 12 seconds).
The main feature is the long distance Xerath can harras, but be carefull to not waste mana!!! Is easy to KillSteal because pretty all your ability have a delay between casting and shooting.

Passive grants armor, but not health, so be carefull! Play Xerath is not easy because of positioning knowledge is required, enemy can evade if know what to do. Also he isn't an aggressive one, but can perform nice gank even if slow.

Take a look to Xerath and give him a try.

Hope that helped you on chosing or not chosing some champions. I'll had more in future if liked. If not please tell me why so I can improve myself and this guide xD

Remember one last thing: choosing a champion for the game is determinate (even) by your teammates, so try to communicate =) And experience against Bot ^^

Have a nice day ;D