Pros:
Strong laning with this rune setup
Good split pusher
Great map control with shrooms
Big nuisance to the enemy team



Cons:
Very hard to master
Get ganked and camped most games
Dies to ganks easily
Weak champion (still getting nerfed)
Poor teamfighter

Key things to focus on:

1. Farm
2. Trading/Poke
3. Map awareness/Vision
4. Minion wave management
5. Split pushing

Farm:
for teemo last hitting is very easy so you shouldnt be missing any minions in lane. practice heaps if you cant cs because on a champion like teemo gold is very important and items in this build are quite expensive and will require a 9/10 cs per minute to achieve full build at 35 minutes. aim for 90 cs at the first 10 minutes and 200 by 20. FARM IS THE MOST IMPORATNT THING FOR TEEMO. AND MOST CHAMPIONS IN GENERAL.

Trading:
depending on what champion you are fighting will determine how you trade. first we will look over melees. for champions like shen and maokai who are very tanky you want to auto q every time they walk up to farm. always look at your minions health to see when the enemy will go to cs. you want to get damage onto melee champions without giving them a chance to fight back and retaliate, as most melee champions can out trade you if they are in range to use their abilities. when you reach level 6 you can throw a shroom onto the minion wave next to where your lane opponent is standing to trigger the explosion onto them. this is a great way to start a trade as they are forced to all in you or slowly walk away while taking damage from your autos. for ranged champions such as jayce and kennen it is a lot more reliant on character movement, dodging skillshots, standing behind minions, and waiting for their cooldowns before fighting. see the character matchup section for tips against each champion. Overall teemo has a simple trade with no skillshots other than the ultimate. the only thing to know is when to trade and when not to.

3. Vision
teemo is a good champion for learning map awareness and placing wards because if you dont look at your minimap every few seconds you will feed your *** off to the enemy jungler and top laner. place shrooms in the river and bushes to use as vision and a slow if the jungler tries to gank you. before level 6 you should be extra careful as you need to rely on trinket cooldown before pushing the wave or going in for a fight. you should also take note of the enemy mid laner location and look to see when the enemy jungler ganks other lanes to see when it is safe to go in.

4. minion wave management.
this is basically manipulating the minions to make them push or freeze how you like. in general you want to push against champions that cant farm for **** under tower or if the enemy jungler is somethign like a shyvana and wont gank you early. otherwise you want the wave to push onto you so you are in a safer position and opens the enemy uip for ganks from your jungler. if the enemy is auto attacking minions the enemy minions will kill yours quicker and push to your side. if the enemy is passive however you can set yourself up for a bad trade by walking into minions, let them auto attack you once and this will drag your minion aggro onto them, while your minions will be attacking the enemy the enemy minions will be killing yours and the wave will push to you. to freeze a lane on your side you want to keep about 3 to 4 minions from the previous wave. smart enemies will try to complete the push to the tower to break the freeze. when this happens you should push back so that the enemy minions are killed before they reach your tower. when the lane resets you can do it all over again and set up a gank. always push the wave to the enemy tower when he backs.

5. Split pushing
teemo can split push very effectively. you want to place your mushrooms in the way of routes the enemies will come to gank you. DO NOT place shrooms in the middle of a path as most people arent stupid and will huge walls to avoid shrooms. when you are split pushing in between waves press S to trigger your passive. if people come you will be invisible and they wont be able to do anything. never go invisible in bushes as this is obvious and if you disapear out of the map the nearest bush is the first place the enemy will check. make sure you purchase control wards to make sure the lane you are split pushing in isnt warded or you will be caught out if when you go invisible. when you are invisible use this time to look at other places on the map and see what is going on with other players. reveal out of stealth when the enemies show up elsewhere on the map. since teemo is a poor teamfight your strategy should be to split push however its only effective when you can 1v2 anyone on the enemy team so thats why its so important to win lane and get as much farm as you can. always kill enemy jungle camps when you are on their side of the map.

Frozen Mallet:
This is the first item you should purchase. it helps you chase down the enemy or kite them if they are chasing you. Good health and attack stats makes you harder to kill.

Hextech gunblade:
Once you finish this item and have frozen mallet you get a big power spike and should be able to fight anyone on the enemy team unless they are fed out of their mind. the active on gunblade and frozen mallet passive makes chasing down enemies very easy.

Warmogs:
Once you hit level 14 the warmogs passive will proc allowing you to regen HP super fast out of combat. This combined with frozen mallet gives teemo heaps of health and makes him very resistant to burst assassins. and against tanks/bruisers teemo wins in a long fight anyway.

Rageblade:
This is a late game item that gives teemo lots of stats and helps him destroy towers and champions in duels late game. Not much else to say, just overall great stat booster. only get this before warmogs if you are super ahead and can already 1v2 anyone on the map.

QSS:
Final item for teemo, you want the first part of QSS earlier if they have hard cc you need to dodge, but the finished item should be the final part to your build.

Boots:
Ninja tabi - Good against AD/ auto attack reliant champions
Mercury Tread - Good against CC/AP champions
Swiftness - Situational boots, get this if u want to make it easier to dodge skillshots

there is only so much i can write about teemo to teach you about him. 90% of how you will improve is playing the game and getting used to the build path and strategy. over time you will learn when you are strong when you are weak and how much damage you will be able to dish out, being able to calculate trades in advance and know how quickly you can eliminate enemy. If you have trouble against certain matchup as teemo. play that champion for 3 games i guarantee the next time you face them you will have a much better idea of their cooldowns, damage, and toughness.