The fact that your puppy hides behind you is not good. A 12 week old puppy should not be aloof...it should want to go to strangers and check things out. You need to work on getting your dog's confidence up. This is not an aloof reaction, its is a fear reaction. This is not typical.
You need to show your dog that strangers aren't bad. Get your puppy comfortable and get it to play/do something with strangers where its not realizing that its even with a stranger.