Formal:
* J'ai beaucoup de devoirs. (This is the most literal and common translation)
Informal:
* J'ai plein de devoirs. (This means "I have a lot of homework" in a more colloquial way)
* J'ai un paquet de devoirs. (This is another informal way to say you have a lot of homework, literally meaning "I have a package of homework")
* J'ai trop de devoirs. (This means "I have too much homework")
Other Options:
* Je suis débordé(e) de devoirs. (This means "I am overwhelmed with homework")
* J'ai un tas de devoirs. (This means "I have a pile of homework")
The best option for you will depend on the context and your relationship with the person you are talking to.