Summarize the following:
"¿Cómo estás?" is the most basic way to say "How are you?" This phrase is appropriate in almost any context when talking to almost any person, whether you previously knew them or not. However, there is a more formal version, used particularly in Central American countries. Note that there are two verbs in Spanish that mean "to be:" estar and ser. However, estar is used in this context because it implies a more temporary state, whereas ser is used to talk about a permanent condition or state of being. There really isn't a formal way to say "How are you?" in Spanish. However, "¿Cómo está usted?" is used, primarily in Central America, when talking to someone who is older than you or in a position of authority. It also doesn't hurt to use this phrase when talking to an adult you've never met before, as it conveys respect. If you walk up to a group of people and want to greet all of them, you might ask "¿Cómo están?" The verb is conjugated for the plural "you," so this is similar to saying "How are y'all doing?" or "How are you guys doing?" in English.  To make this question informal, ask "¿Cómo estais?" (koh-moh ehs-tah-iss) Keep in mind that in some Spanish speaking cultures, it might be considered more polite to ask this question of each person in the group individually, rather than addressing the group as a whole.

Summary:
Say “¿Cómo estás?” (koh-moh ehs-tahs) in most situations. Use "¿Cómo está usted?" (koh-moh ehs-tah oos-tehd) when you need to be more polite. Ask "¿Cómo están?" (koh-moh ehs-tahn) if you're greeting multiple people.