While you can say that, in general, ‘tú’ is used for informal situations and ‘usted’ is used in formal situations in Spanish, this rule varies wildly depending on where you are. In Colombia, for example, it is not uncommon for families or even couples to use ‘usted’ with each other; while in Spain ‘usted’ can even be slightly offensive because it introduces too much of a distance with whom you’re speaking. Just try to adapt to the context you’re in, and if you get wrong, no one is going to judge you for using the wrong pronoun, so don’t sweat it!