Well, hello Summoners, this is my first build guide, so I hope I don't screw up too much. I'm going to introduce you my Tristana AD build, since she has been my main ever since I started, and has seen me through 200+ battles.

Tristana is an excellent AD carry, who is also pretty overlooked. Her W lets her get the best position always, while her ult is pretty good at opening gaps. She also has an in-built Ignite qith her E, meaning she can skip Ignite, or use both Ignite and her E to add up tons of damage early game.

Now this is the point that REALLY makes Tristana a game-breaker if played well: she is a carry, meaning she'll dominate late game, yeah, but she's starts pretty much like an assasin, allowing her to own her lane early game, and even get some kills.

For runes I go full offense, with Armor Penetration Marks and Quints, and Attack damage per level Seals and Glyphs. Many won't agree with this last choice, seeing how I give up almost completely in sustain to get just a little bit more damage, but I really don't think the tiny sustain I can get from defensive Runes is worth it.

Again, full offense. Everything is pretty much self-explanatory, save a few things.
Life steal: I feel this is actually a very important mastery to get, since it buffs my life steal to 6% along with Doran's blade. That's enough sustain for laning, while 3% falls pretty short. Once I get Vampiric Sceptre I get 18% life steal, which is close to 20 health per second when attacking minions.
The utility tree: I really want the speed buff, and Trist really needs some more mana, which I can get along the way. The defense tree is pretty much useless, as 6 armor WILL get penetrated by the enemy carries and 4-5 more damg per hit early game do not matter.

I open with Doran's blade for sustain (LS), survivavility (100 Health) and some extra damage. Then I go for the level 1 boots and the vampiric sceptre. That lets me farm easily, as long as I'm careful. Then I go for the Pickaxe and the Bzk boots, trying not to recall till I have enough money for both. After this, I can start farming in the jungle (if my team doesn't have a jungler that needs the creeps, although with the new jungle, the respawn is so fast I can kill them anyways most times) between minion waves, farming as much as I can. On next recall, I want to have my IE ready, or at least the B.F. sword, which grants me much needed AD. With the IE around, its time to try and get some kills. After it, the phantom dancer to increase Crit %, and with that i have finished my core build: Doran's blade, Bzk boots, Vampiric sceptre, IE and PD. This is more than enough to end some N00B games. And its also enough to kill baron, if your team's not doing too bad.
After these items, its time to increase your AD, so go for Bloodthirster. At this point, most enemys should drop in seconds before you, so you can start with the situational items:

Last Whisper: Excellent item, gives much needed Armor penetration: 46% + 31 (approx) means tons of damage to everyone. Its also cheap, and grants some more AD too.
Black cleaver: Pretty good too, better than Last Whisper, but a little more expensive, depends on how you're doing. I'd rather pick this if I can.
Starks fervor: If your team has another AD carry, you can pick this, but it shouldn't be your 1st choice.
Phantom dancer: A second PD is a great option. Yes, it grants very nice AS and Crit%, but the best thing is its Movement speed. I think this is a more defensive item than a frozen mallet is, since it allows you to outrun most threats
Bloodthirster: If you need even stronger damage, this beauty will send you through the roof, while also granting 56% life steal. Crazy.
Warmog, Thormail: If there's an annoying AD carry dropping you, this will help you. but only if you don't need any extra damage.

WHAT NOT TO GET:
Madred's Bloodrazor: This sucks on Trist. Armor, which I don't need, AS, but not too much, and a pretty much useless passive. 4% magic damage. Meh. Think of a tank. Say he has 4k health and 300 armor, 150 MR. Madred's would hit for 4000*0,04= 160 magic damage. Since we have no MPen marks, its 160*(100/(100+150))= 64 damage. Meh. If you get Last whisper, and almost halve his armor, you'll get so much more damage.
Trinity: So expensive. Also has many stuff I won't use, such as mana and AP (I don't need mana late game, though I did early game). Also, its passive won't get used, since I won't be skill-spamming anytime soon with such long cds.

Most Trist builds pick a 1st point in E, instead of W. Why? I always found it better to get my escape/chase tool right off the bat, rather than waiting till level 2. If I want to try and get a kill, the magic damage (instant, not DoT) and the huge 60% slow will give me a chace, while the Explosive shot won't. Also better for farming, since I can deal AoE damage to minions.

That being said, you first go for Rocket jump, then Explosive shot, trying to get as much in Rocket jump as you can, till level 6. After getting your ultimate there, inmediatly max out your esteroid, Q, Rapid Fire, so you can deal tons of damage in the soon-to-begin teamfights. After this max Rocket jump, to reduce its cd, and finally Exp. shot, since we won't really care about the damage in it, just the grievous wound, at this point of the game.

Ignite adds up to your E, dealing really bid DoT, which means you can jump, auto-attack, Ignite and Explosive shot, auto-attack, then run away safely (60% slow) having dealt crazy damage (over 300, at level 2!!).
Flash sinergizes well with your W, and helps you escape almost any gank.

Other options:

Teleport: for backdooring. Its nice.
Revive: Hell no.
Heal: Heaven no.
Clarify: Good sustain early game. Useless afterwards, so no.
Exausht: Nice sinergy with the slow on the jump. Its ok.
Promote: Not for you.
Clairvoyance: Not for you either.
Smite: Excellent choice, lets you jungle and kill Baron at level 1.
Surge: Nice sinergy with your Q, has however long cds. So again, no.
Cleanse: If they get you with CC, you won't last enough to click this spell, so no.
Ghost: If you need even more escape, but you should have enough with W and Flash.

Pros:

Excellent farmer, due to her range
Can play assasin at early game, then turn into carry
Best positioning champ
Great at gap-closing/creating

Cons:
Squishy!
Needs careful playing, not for begginers
Item-dependant
Can be screwed if you're laning with a poor player
Weak ultimate

And that's it. My first attempt at a champ guide XD.

Tristana, a solid farmer that can assasinate early game, and get the gold going to her pocket.