The difference between tu and usted often described as grammatical, but it's also in large part social. It's a bit like asking whether you should call someone ma'am or sir, or buddy. It depends on the social context. As mentioned in other answers, the broad outline has to do with age, social status, and familiarity, with tu being less formal. However, the important thing to understand is that this context varies across different countries and situations.
To give one example, in Spain, there's a bit of a trend to call basically everyone tu. There are still plenty of people who will call you usted, particularly if you're a customer or they're older (thus more likely to be traditional), but the use of tu is growing. For instance, my lawyer, who's 10-20 years older than me, prefers to be called tu. One example of this is that there have been several controversies in recent years about tutear al Rey, addressing the king of Spain, who is over 50 and of very high social status, as tu. A journalist did this some time back, and faced criticism from traditionalists, but even the leader of a left-wing political party did so as well. Even though they faced criticism, the fact that they did so is a sign of how far the phenomenon of tuteo has progressed in Spain.
As another example, at least in Colombia there exists phenomenon in which usted is used to indicate familiarity. As the thesis El ustedeo de familiaridad en Colombia and other research indicates, depending on factors such as gender, whether usted or tu is preferred for an informal interaction can vary.
Overall, it's probably best to ask what someone prefers if you're not certain. Sticking with usted is safer, since no one minds being treated with politeness, but it's not perfectly risk-free: some young people might feel like you think they're old. If you have the time, investigating what the usual usage is in the country you're in can also be helpful.