Formal:
* Comment allez-vous ? (This is the most formal way to ask how someone is doing.)
Informal:
* Comment ça va ? (This is the most common way to ask how someone is doing.)
* Ça va ? (This is a more casual way to ask how someone is doing.)
* Comment vas-tu ? (This is used with someone you know well and is more informal than "Comment allez-vous?")
Other options:
* Quoi de neuf? (This literally translates to "What's new?" and is a casual way to ask how someone is doing.)
* Tout va bien? (This means "Is everything going well?" and is a more formal way to ask how someone is doing.)
The best way to choose the right phrase depends on the person you are speaking to and the situation.