How to dress. You should always wear what you feel comfortable in, don't just wear something to impress you crush or fit in. That being said, you should at least make an effort not to look scruffy.
How to talk. Never, ever, talk funny in front of your crush like a lot of people do. How you talk to him or her makes him or her think bad or good, weird, or nice about you. For example, if you talked to a boy like "um, um, hi___, pretty nice weather, huh?", he would think that you are weird, and then he would find out eventually that you have a crush on him because of the way you talk around him.
Act impressive around your crush, just don't act too fake.