CLARO Q SI UD ES FORMAL Y TU ES CUANDO YA CONOCES A LA PERSONA FAMILIA ,AMIGOS, ETC

Usted es formal y (vos) se utiliza cuando conoces a la persona. Cuando conoscas a la familia obviamente lo mejor seria llamarlos por sus nombres.

It depends on the situation and the country. For example in Spain (where I live) you should use Ud. when you want to be very polite, but if you are talking with a friend or someone of your family, coworker, etc. you can use Tú. In some regions of Spain and in other countries it's very usual to use always Ud, and Tú is considered not very polite. I hope to have helped you. If you want to practice spanish we could try to put in contact using Skype (pacoandrades72), I'm looking for someone to practice english too.

Hola! Soy de Argentina! Podes usar usted al principio hasta que ellos te den permiso para que los trates de vos.