Formal:
* Comment allez-vous ? (This is the most formal and polite way to ask "How are you?")
Informal:
* Comment vas-tu ? (This is a more informal and friendly way to ask "How are you?")
* Ça va ? (This is the most casual and common way to ask "How are you?" and literally translates to "It's going?")
Other options:
* Comment allez-vous aujourd'hui? (How are you today?)
* Comment allez-vous ces temps-ci? (How are you these days?)
The best option to use depends on the situation and your relationship with the person you are speaking to.