Formal:
* Je vais bien, merci de demander. (pronounced: juh vey byan, mer-see duh duh-mahn-day)
Informal:
* Ça va bien, merci ! (pronounced: sa va byan, mer-see)
More casual:
* Je suis bien, merci ! (pronounced: juh swee byan, mer-see)
You can also use these variations:
* Tout va bien, merci ! (Everything is going well, thanks!)
* Pas mal, merci ! (Not bad, thanks!)
The best choice depends on the context and your relationship with the person you are talking to.