In Spain we almost always use tú. Usted is pretty much like saying Sir or Ma’am in America. The most common place you’ll hear Usted is in bars, restaurants, grocery stores and other businesses.  The bartender, waiter, checkout person, etc. will address you, the client, as Usted. The other common place to use Usted is with people beyond middle age. Regardless of the situation, it’s customary to address a person age 60+ as Usted. Chances are you’ll rarely if ever have to use the Usted form during your time in Spain. So, don’t worry too much about it. Even if you use the tú form when you could have used Usted, no one is likely to feel offended.