Formal:
* J'ai oublié de faire mes devoirs. (This is the most literal translation and the most formal way to say it.)
* J'ai oublié mes devoirs. (This is a slightly less formal way of saying it, but still appropriate in most situations.)
Informal:
* J'ai zappé mes devoirs. (This is a very informal way of saying it, meaning "I skipped my homework.")
* J'ai oublié mes devoirs, j'ai la flemme! (This is even more informal and means "I forgot my homework, I'm too lazy!")
You could also say:
* Je n'ai pas fait mes devoirs. (This means "I didn't do my homework," which is a bit different from forgetting.)
* J'ai laissé mes devoirs à la maison. (This means "I left my homework at home.")
The best way to say it will depend on the situation and who you're talking to.