First of all Teemos claim to fame is his move speed and kiting abilities, early on Teemo is the least mobile and the least omnipotent (he has the least map awareness) ,the biggest way to cripple a Teemo in top lane is to gank him while he is level 4 / 5 (or even level 2 if your being really mean) since as soon as Teemo hits level 6 your chances of ganking him are significantly reduced and you may even have to lane switch or even have your jungler babysit top.
Another way to counter Teemo is to out harass him during the laning phase, now only a very select few can actually pull this off (such as Kassadin and Mordekaiser), but the more levels you allow Teemo to attain, the harder he will zone and cripple your farm.
The other major way to counter Teemo is by gaining insane map awareness, keeping track of his shrooms and his position, this way you can intercept his favourite offensive shroom placements and punish him for being brave, an example of this is to closely watch when Teemo turns level 6 and make sure your jungler is in a brush that Teemo is about to shroom in, this way he will literally facecheck you, and he is highly likely get pwned.
Oracles elixir will also change how a Teemo lays out his shrooms and even for how long he farms, as with oracles you completely cut out his map awareness and make him vulnerable, most Teemo's will begin closing their perimeter of shrooms which creates a minefield on the lane, however since most junglers are highly mobile assassins it shouldn't become much of an issue to gank Teemo once this occurs.
Also don't forget that Teemo is not actually the best damage output in a team fight, the carries (both AP and AD) will do way more damage overall in a team fight as Teemos build will usually be more resilient, and several Hybrid builds have a small pool of mana, and even damage wind-up effects such as Guinsoos Rageblade. Dont forget that my Hybrid Teemo deals larger portions of Magic damage rather than Physical due to Toxic Shot and a large array of items, if he is going out of control invest in magic resistance!
Switching targets in quick succession onto a Teemo will catch him off guard and away from a shroom, this allows you to quickly close the gap and prevent him from kiting your team once his team begins to bail.
Teemo is highly reliant on harassing during the laning phase, however he has a short attack range of 500, which allows you to throw stuns on him as he runs slightly towards you, such as Tarics dazzle or Alistars 1-2 combo which can also throw him off guard and cause him to be more reserving of his little darts.
Ganking Teemo before level 6 will usually tempt him to go for more resilient items early such as a Wits End or Merc Treads which can reduce his damage output ever so slightly, early on.