Formal:
* ¿Cómo está usted? (This is the most formal option and should be used when addressing someone older than you, someone in a position of authority, or someone you don't know well.)
Informal:
* ¿Cómo estás? (This is the most common way to ask someone how they are, used with friends, family, and people you know well.)
Less formal:
* ¿Qué tal? (This is a more casual way to say "How are you?" and is similar to "What's up?" in English.)
Even more informal:
* ¿Cómo andas? (This is a very informal way to say "How are you?" and is only used with close friends.)
It's important to choose the appropriate level of formality depending on the situation.