As Teemo is a ranged champion, expect him to try and harass you as much as possible. Investing in defensive starting items could be a good way of dealing with him from the get-go.
Teemo will place Shrooms R around the edges of the lane. Avoid walking to the side of the lane and only walk the path your minions take to reduce your damage taken.
Be prepared to sacrifice some CS for XP and health. Teemo has a lot of kill pressure if he takes Ignite and is able to harass and poke you down.
Teemo may not join team fights as he isn’t very good in them. He may instead be split pushing and trying to take objectives in the side lanes. Make sure someone is able to stop him from splitting.
Teemo will place Mushrooms R around objectives and in high traffic areas. Avoid going anywhere that Teemo has recently been in without a Sweeping Lens. He and his teammates may be ready and waiting and try and pounce on you.
As Teemo will be split pushing or away from his team during fights, you could use the numbers advantage to force a fight 4v4 or 5v4. Just keep an eye on him and be prepared to rotate if he gets close to your tower.
At level 6, Teemo’s pressure increases thanks to his Ultimate R giving him extra damage and the ability to place Mushrooms R all around the map.
At 40% cooldown reduction, Teemo is able to play a lot of Shrooms R around on the map. Try to take unorthodox pathing to prevent walking through them.
Nashor’s Tooth is a core item for Teemo. It gives him lots of cooldown reduction and damage- which makes him more of a threat.