From my experience, you will only switch to Du if the other person invites you to. I learned German 25 years ago, and I still use Sie with most of my in-laws' friends. You also use Sie for strangers, cashiers in the store, waiters, etc. You use Du with family members and close friends. Students generally use Du, but teachers are always Sie. When in doubt, use Sie. If the other person wants to switch to Du, they'll tell you.

YOU=DU SO YOU CAN ALWAYS USE DU FOR EVERBODY THE PEOPLE THAT GET OFFENDED BY THIS USE ARE FREAKING DUMB CUZ DU REFERS ALSO TO YOUNG PEOPLE AND TO BE CONSIDERED YOUNG IS ALWAYS GOOD

"But my question is what will they say to me when they want to use Sie" - They won't say anything. They'll just say "Sie" and hope that you'll notice and also switch to "Sie". Because saying "Bitte duzen Sie mich doch" is being friendly (if not actually offering friendship, but that's what it used to be in my mother's time), while saying "Bitte siezen Sie mich doch" means keeping somebody at arm's length and therefore feels a bit like an insult.

That can be a very complicate affair :) If they address you with "Du", it's pretty safe to answer back. Unless they are your teacher, or an older person commanding respect. For people your age (and yes, I checked your profile pic) - you're pretty safe to use Du. If you've been going with the formal Sie for a while, it can get pretty awkward. Normally the older person offers to say "Du". My 75 year old mother has a new partner - he is still saying "Sie" to me - I address him with "Du" - 20 yrs of living abroad knocked the formalities out of me. It's become very odd now, because he can't snap out of it, and I refuse to get drawn into it. I offered, but of course it's not my place to offer. So we continue with our weird arrangement. Books could be written about this topic. Just go with what feels right. Stiff and crotchety - definitely use "sie". If you travel and stay in hostels, go with "Du", you'll be fine.