First, if your just doing this because you want to be cool, stop right now. If you don't, and you are new to the skateboarding world, read this. Hope this helps!
Step 1: The Clothes
Be yourself. Don't wear skate clothes all the time because you might be called a poser. Just wear a simple shirt with a logo, some jeans, and some skate shoes. You'll need rubber soled shoes. And not DC's.
Step 2: The Talk
Skaters call guys "Dude" or "Bro". For girls, "Dudette" or "Bra". Get used to this.
Step 3: The Equipment
Just one thing about your skateboard. You would rather have an expensive skate shop board than a cheap toy store one. And pick out some nice, comfortable pads. You won't look cool, but being safe is always better. Only let the pros wear no pads. They can handle it.
Step 4: The Skill
Try your hardest at skating. Practice and practice until you master a trick. I would start out with a ollie, the basic movement to unlock a whole world of tricks.