THIS is my first guide so please go easy on me.
Teemo is, as his title suggests, a swift scout, in this build we will be mainly focusing on this. Teemo is very good at ganking mid-late game if you have a lot of attack speed and damage. You should try to get at least 1.8 or more attack speed and over 130 damage. Don't try to go in alone as you will die very quickly. Try to get a teammate to push an enemy back and wait behind them then attack. I will go into more detail about this later.

This guide can be used in either 5v5 or 3v3 but in 3v3 there is no blue buff.

THE runes I chose were mainly attack speed and damage. The reason for this is when trying to gank as Teemo, you really want to get a lot of damage done in a short amount of time. The runes provide quite a bit of attack speed so at lower levels you have greater attack speed and the same for attack damage.

MASTERIES that I chose were life steal, armour penetration, attack speed, attack damage as well as little things like ability power and critical strike. The main parts of the masteries is the life vamp and attack speed.

LIFE steal is good as when you gank early, mid or late game, you might lose a lot of HP as champions might focus on you as you might have the lowest HP. If you gank someone and are low HP you can either recall or just farm minions for a bit to get back your HP.

Attack speed is to increase attack speed (obviously) to get more damage done in a short space of time. The reason for this is say you are setting up a gank on an enemy. They are full hp and your friend gets them on 50% health, they will run back and hopefully they run in your direction. If they do, then you have a short time to attack them as they run, if they have Ghost or Flash they can get away easily so if you have high attack speed you have a greater chance of killing them if say your attack speed is 1.

THE skill sequence is pretty basic for Teemo with you wanting to level up your Noxious Trap every time you can at Level 6, 11 and 16. Your Blinding Dart, you want to max out as soon as you can. After that, max out your Toxic Shot and then your Move Quick.
The main reason for levelling up blinding dart so quickly is you can do a lot of damage and succeed in early game ganks as they miss all their attacks for a couple of seconds.

Your passive is a very powerful tool so is it wisely. Say you are playing a 3v3 and it is early game. People always gank bottom for some reason so as fast as you can, buy something then run into the bush at bottom. If you are lucky you should have enough time to go invisible. If one person runs in try to harras them by using your blinding dart then attack for a bit then run. If successful they should be on maybe 50% Health. The same goes for 5v5 no matter what lane you are playing (expect mid as there are no bushes in decent spots to hide in so just hide in the middle really quickly).

So all in all, Teemo is a very good character for ganks if played properly and if your teammates are decent and don't feed. Try to get some early-mid game ganks if you can to get money and XP quickly.
Thanks for reading this, It is my first guide so I can improve on quite a few parts but tell me what you think and what I could improve on.
-Thanks!