Cuándo se debe utilizar formal o informal?
Estoy muy confuso en este pregunta. Por ejemplo:
Si charlo con alguien en la linea en un banco, debo usar usted o tu?
Si acaba de econtrar alguien, asumo que debo utilizar 'usted'. Si llegamos a ser amigos, cuándo cambio a 'tu'?
Yo no conozco a ninguna personas que hablan español, sólo curioso.
You use "usted" when you do not know a person that is older than you. You use "tu" for family, friends, relatives, God, people that you do not know but they are younger or approx. the same age as you. Of course, you always use "tu" for kids. I actually use "usted" when I talk to my parents. But, that depends on the family. If after you use "usted", that person tells you that you can "tutearlo/a" then you can use "tu".
This is extremely dependent on region. In Venezuela I very rarely heard usted used by anyone. In Argentina brothers called each other usted. In most places you use tú with friends, family, children, pets, God, etc. You use usted with everyone else. But Leslie is correct in saying when in doubt, use usted. If someone uses tú back to you, then you can switch.
Sylyon and KevinB are right. In Spain they always use tú. Sometimes if a parent is angry at their kid they will use usted. For example, the first time the parent tells the child to clean his room, he uses the tú command. Limpia tu cuarto. The second time, he uses the tú again. Limpia tu cuarto, hijo. But the third time, he says, "¡Limpie usted su cuarto!" This is just an example. :D. It's not a hard set rule.