Despite much prejudice around the idea of Spanish supposed rudeness and lack of manners , actually, Spanish can be as polite as they need to be though they express it in a different way than, say, the English.

The Spanish "usted" form is mostly used in formal contexts or when you appraoch someone you don't know like, for instance, when asking for directions of for help. However, it is equally important to mention that the use of "usted" in certain contexts can be interpreted wrongly as a sign of superiority or as a sign of unnecessary distance. So, don't treat someone you already know, a friend, as "usted" in an informal context. However, use "usted" with someone in a higher rank, in a job context, or an elderly person unless instructed otherwise. Even me, a Spaniard, have found myself taken aback when treating 50 or 60 year old people as usted, they usually say "why are you treating me as usted?, do I look so old?

The best strategy is to pay attention to what the person wants to be treated as Spanish will probably let you know very soon if you are allowed to treat them as "usted" or if they want a more relaxed and informal kind of chat using the form "tu". It all depends on context and the situation.

Finally, to use the "usted" form correctly always take the third person singular/plural form of any verb to address a "you", that's how it works.