Formal:
* Je suis professeur de français. (This is the most common and formal way to say it.)
Informal:
* Je donne des cours de français. (This literally translates to "I give French classes.")
* Je suis prof de français. (This is a shortened, informal version of "professeur de français.")
Other options:
* J'enseigne le français. (This means "I teach French" and is a bit more formal than the informal options.)
* Je suis enseignant de français. (This literally means "I am a French teacher" and is slightly more formal than "professeur de français".)
The best way to choose the right phrase is to consider the context of the conversation and the person you are speaking to.