Formal:
* Je vais bien. (I am well.) This is the most common and polite way to say you are doing well.
* Je me porte bien. (I am doing well.) This is a slightly more formal way of saying "I am doing well".
Informal:
* Ça va. (How are you?) This is a common, informal greeting, but it also implies that you are doing well.
* Je vais super bien. (I am doing great.) This is a more enthusiastic way to say "I am doing well".
* Je suis en pleine forme. (I am in top form.) This implies that you are feeling particularly well.
Other options:
* Tout va bien. (Everything is fine.) This is a more general way to say that things are going well in your life.
* Je suis heureux/heureuse. (I am happy.) This is a way to say that you are feeling good emotionally.
The best way to say "are doing good" in French will depend on the situation.