Hey there !
Use "vous" when...
a. you are speaking to more than one person. Therefore, if you speaking to two or more people at once when inquiring "How are you?", you would say "Comment allez-vous?"
b. you are speaking to someone in a formal manner. For example, if you are speaking with a professor, an elder, or anyone else with whom you are not on familiar terms. The vous form is more respectful.
Use "tu" when...
a. speaking to a single person with whom you are on familiar terms. For example, when addressing a friend or a sibling.
b. given permission to switch from the vous form. For example, a professor may say that you can use "tu" instead of "vous."
Vous: plural and/or formal
Tu: singular and familiar or with permission from someone with whom you would normally use "vous"
If you are unsure if you should address someone formally or not, I would err on the side of caution and use "vous."
I hope this helps !