If I were you I would start with picking a store, say American Eagle and looking around and just pick up the items thay seem "cool" to you. Before you decide that you like that item you should check for quality, like how thick the material is and how well it is put together. You can check that by looking at the seams and seeing how they were sewn. If they are thin and strands of string hang out then the item will not last long. When you have several items and are trying them on you should try using the items in outfits. Like instead of tring a t-shirt on and then trying on jeans try them on together to see how they each would work  in an outfit. I would also start with basics like t-shirts with simple designs with kahki shorts or jeans and sneakers or flip-flops. That style shows a care-free, easygoing attitude, if that's what you're going for. Also I am a girl so you might want to ask another dude about my advice before using it just to stay on the safe side.