Look at it this way. Use "tu" when you are talking with your family, friends, people why are younger that you and basically anyone you are familiar with (that is meant my "tu is informal", it is used in informal SITUATIONS).
Use "Usted" when you are talking to your teacher, you neighbor, the president of the country, people you see for the first time, people you know (but are not familiar with, that means they are not your friends), but are older than you and basically anyone you want to show respect to.
BUT!!! For example, in Colombia, they use "usted" all the time... With friends, teachers, neighbors, etc... And they use "tu" almost never. In Argentine, instead of "tu", they use "vos" and the verbs for "vos" are conjugated for the second person plural "vosotros" without the 'i'.
Vos sos - you are
Vos tenés - you have
Vos hablás - you speak
Vos pensás - you think
So, you need to learn the culture of the country to speak the language :D
Hope this helped a bit, and if you have any more questions, feel free to ask me directly ;)