There are four ways to say "you" in Spanish:
tú - singular, informal
vosotros - plural, informal (only used in Spain)
usted - singular, formal
ustedes - plural; technically formal, but used for formal and informal in Latin America (since we don't use "vosotros")
(Some countries, like Argentina and Colombia, also use "vos," but that's probably not taught in many textbooks.)
If you are talking to one professor (or stranger, or authority figure, etc.), you would use the "usted" form of the verb (¿Cómo está').
If you are talking to two or more professors, you would use the "ustedes" (plural) form (¿Cómo están'); however, if you are talking to two or more friends (and you are not in Spain or talking to Spaniards), you would use the "ustedes" form as well, because you need a plural form.
When talking to one friend, you would use the "tú" form of the verb (¿Cómo estás').
I hope this makes sense and answers your question.