This is one of the biggest challenges for beginning Spanish speakers. Why do there have to be so many ways to say "you"? Basically, Spanish makes several linguistic distinctions that we do not make in English.
In standard English, we differentiate between singular (one person) and plural (2+ people) in first person (I, we) and third person (he/she, they), but not with second person (you). We could say "You speak Spanish." to one person, or to an enormous group. Southern US dialects use the non-standard "y'all" to talk about a group of people, and Spanish is quite similar. For one person, you should use the singular forms tú or usted. For more than one person, the plural forms vosotros, vosotras, andustedes are correct.
In English, we use "you" for everyone: friends, family, bosses, teachers, our dog, the president. In Spanish, you have to be more careful to be polite. If the person you are speaking to is significantly older (e.g. a grandparent), if you have a formal relationship (e.g. your boss, a client, or a customer), or if you don't know each other personally (e.g. a stranger), you need to use the formal pronouns (usted and ustedes). If you are friends, family, or if you are speaking to a child, you can use the more familiar forms (tú, vosotros, vosotras). If you're not sure which to use, err on the formal side, but listen to what the other person says and follow their lead.
The vosotros and vosotrasforms are only used in Spain. In Latin America, ustedesis used for any plural group, regardless of level of formality. In Spain, you must use vosotrosfor males or mixed groups you are familiar with, vosotras for female groups you re familiar with, and ustedesfor groups in formal situations.