This is my first ever guide but I've been running this Teemo build for a while and very rarely do I ever go negative so please try it out before you downvote. :)
P.S I have put a lot of stuff into my notes and so make sure you look through those.

Pros Cons
-Easy to farm with -Ult can use up a lot of mana
-Good escape ability with move quick -Very squishy and can often easily be
-Poison OP targeted
-Passive allows for sneaky plays -Drops off late game
-Noxious trap is a way to get yourself -Not great burst damage
into a good position
-Counters a lot of top laners

Teemo is a lot different than most mid/top laners so first 3 levels you can often dominate the lane and get yourself ahead on cs however after that most champions have some way to engage you so you will have to be more careful. Due to his basic attacks being ranged even if you have been well pushed it still should be pretty easy to get farm. If you get Runaan's you melt the minions and getting cs is no problem.

For spells I tend to stick with the usual flash and ignite. If you are solo top fitting teleport in there wouldn't be bad. Clarity is a good option if you find yourself consistently oom with him and barrier can be used on a teemo to keep yourself alive when running away. For the most part I'd say definitely stick with flash but due to his poison ignite isn't totally necessary.

Teemo is often abused in the League community and for a good reason. His poison can be frustrating for teamates as there is the potential for a KS however usually I'd rather be with a Teemo than against one. Teemo can put himself in very good positions by going invisible behind an enemy and then ambushing when they try to run away, this gives him good ganking potential. His shrooms can save many lives by blocking paths and allowing allys to have a place to run to when they are escaping.

Teemo's passive aswell as giving him good ganking potential can help him in lane going invisible in turret range can give you a lot of damage against an opponent with no repercussions. He can also shroom an area and wait for someone to wander in and potentially pick up an easy kill. His shrooms can also be really usefull as they can essentially act as wards so you can put them around baron and dragon to prevent an easy capture of one of them. In team fights Teemos blind is best used on the enemy's adc and just with that one attack he can prevent their main damage dealer from dealing damage an single-handedly tip a team fight

Overall Teemo is a very versatile champion and can fit many roles. He also is able to snowball effectively and is capable of carrying a team. Teemo is a fairly simple champion and can be a good buy for a beginner.