Should I use, "Tú" or, "Usted" When Talking to People I Don't Know?

Should I use, "Tú" or, "Usted" When Talking to People I Don't Know?
 

I know tú is informal, and usted is formal. So, when talking to a stranger, should I use usted?

I'm wondering this because I'm traveling to Spain in a few months. My Spanish speaking skills are at a beginner level, but I would like to address people properly. So, for instance, if I'm in a store and I ask one of the employees in the store a question, should I use usted with them instead of tú since I don't know them well?

All the explanations I've seen for these two words have confused me, and I'm hoping someone can help. I don't want to offend anyone by saying tú when I should say usted, or vice versa!

Thanks!

You may want to take a look at an old discussion about it here.

Thanks! I read the discussion and I understand now.

Trust your intuition. You'll do well. ;)