Don't try to show what an original free thinker you are by wearing horrifically garish and outdated clothes, you will just look like a clown and people will not wish to be seen with you.
Do not show off how rich you are by wearing expensive designer labels and then sneering about your friend's tesco jeans, you just look like a knob.
Do: wear t-shirts of bands you like or sports teams you support, its a good conversation starter.
Do not: wear t-shirts of bands you don't like or sports teams you don't support, that's just weird.
Do: wear appropriate clothing for the weather conditions and social situation.
Do: wear your rucksack on both shoulders if you find it more comfortable.
Do not: spend an hour agonising over what to wear. No-one really cares what you wear, but they're getting really ****ing annoyed waiting for you to make up your mind.
Do: experiment. There is no better time in life to grow a mohawk or dye your hair green.