In most cases you're going to want to play safe and wait for your first item. Teemo mage support is nice to keep the enemy laner from CS-ing with the blind, but that's about it before level 6. Try and keep them from getting cannon if possible. Mainly the goal in early game is to tilt the enemy so they under-perform in mid-game. When Teemo hits level 6, spam shrooms with the intent to stop ganks. This should let you get first item without inting. Play heavily around your ADC. If your ADC is good and you have a lead, Teemo's poison can do a lot of damage, especially with Press the Attack.

Heal heal heal. You have Moonstone which heals every 2 seconds as long as you're damaging the enemy, which is very easy to do with Teemo's poison. Keep playing the your ADC and keep them alive. Focus on saving your ADC as much as possible. Get your ADC fed. Remember to play support and don't go for kills by yourself because you won't have the kill potential to do much on your own. Play for team fights and try to stay alive for as long as possible while poking the enemy to heal your team. Spread your shrooms around areas where team fights happen, and in places where they will be irritating for the enemy. Mid lane and right outside there base are my go-tos. Play around with how much healing you can actually do and make sure you don't overextend.

Nothing much different than mid game. Your heal will be fantastic in team fights.

Essentially the same thing as mage Teemo, but you have to execute minions to get gold and poke for Grasp stacks. Keep blinding the enemy ADC. Mid game you will have more sustain in team fights and you will be able to get more in the middle of your team to heal everyone. You have Grasp and Black Cleaver instead of Press the Attack, so you will be shredding armor for your ADC instead of increasing all damage dealt to the target.