To try and fit in with other people, you need to start by acting normal around them. You also need to be confident and follow the leader. The way you look really matters, therefore you have to dress to impress.
First off smile. Not a huge goofy grin, just a genuine soft smile. Act confidently. Sit or stand straight and make eye contact. Ask about them and actually listen to their answers. As for the dress to impress, you don't have to go over the top with that. Wear things that are clean and not wrinkled, and that fit. Make sure your hair is brushed. Where the clothes come from really doesn't matter. I have designer clothes and I have Target clothes. I get just as many compliments on the Target clothes as I do the designer stuff.