Formal:
* Oui, je le ferai. (literally "yes, I will do it") - This is the most formal way to say it.
* Oui, je vais le faire. (literally "yes, I am going to do it") - This is also formal, but slightly less so than the first option.
Informal:
* Oui, je vais. (literally "yes, I am going") - This is more casual and can be used with friends and family.
* Oui, bien sûr. (literally "yes, of course") - This is a very common way to say "yes, I will" in informal settings.
* D'accord. (literally "agreed") - This is a simple and informal way to say "yes, I will."
You can also use the more emphatic Oui, absolument! (yes, absolutely!), or the more casual Ouais, j'vais. (yeah, I will).
Ultimately, the best way to say "yes, I will" in French depends on the specific situation and your relationship with the person you are speaking to.