Nunu's early game is powerful, but not an issue if you play defensively. Teemo, blind is gay. Specifically what kind of problems are you having against them? Here's some hints to help:
Ghost/Cleanse and Ghost/Flash are your two biggest friends. I prefer ghost cleanse since there is so much **** out there that will ream you in a stun/snare/root. Additionally pick up items like Madred's bloodrazor, Sheen > triforce, Mercury Treads, Malady, Phantom Dancer, and Banshee's. Also try building your skills in the order you'll most likely need them. If you're getting hit hard, put more levels in your heal early.
E, W, Q, E, Q, R. That's a basic build that works out nicely. If you do find yourself being hit hard you can swap out your first Q rank for an extra heal rank.
Lastly, a good starting item is Doran's Ring. The health is quite nice, but it;s the 5mp5 that really saves the day.