In an informal, friendly situation, you use "Tú," which is the informal "you." You would generally use "tú" when talking with friends, family, co-workers, and other people you are on a familiar basis with.
The other option is "usted," which is a respectful way to say "you." It might be used with elderly people, such as your grandparents, or with your professor or doctor, or with people you have just met, or if the situation requires formality.
"Usted," like "tú," is singular. When you want to say "you all," referring to more than one person, you use ustedes. Unlike the singular form "usted," "ustedes" does NOT communicate respect or formality. In Latin American countries, it is the ONLY way to say "you" when referring to a group of people.
Note that in Spain, there is a plural form of "tú" -- vosotros. "Vosotros" is used in informal situations, but is not used in Latin American Spanish.
The exact rules for when to use "usted" vary a bit based on the country and region you are in, with some countries being more formal than others. When in doubt, it is probably best to use "usted," as being overly familiar with a person can come off as rude.