Have you ever wanted to just break loose of those chains holding you down? Have you ever wanted to just scream and run and tell someone no. Do you want to be a rebel? This means doing what you want, when you want, where you want.

[edit] Steps

Consider dressing the part. You don't have to dress a certain way, but to really show you're a rebel you might want to wear ripped jeans, or black shirts.
Show people you don't care what they say. This means if someone goes up to you and tells you they hate your shirt, you can just walk away, or say to them "I like your shirt too."
Act like you might want to get arrested or something. Or pull a crazy stunt. Don't take crap from anyone.
Stop serving. For example, if you have been doing the laundry for your thirteen year old upstart, so that he will have clean clothes to wear, just stop. Do what you want, when you want, where you want.

[edit] Tips

Remember that regardless of what anyone else says, the key to rebellion is your attitude and not your dress sense. You can rebel just as well wearing a Disney Princess costume as if you were wearing something that looks as though it's been plucked from the Sex Pistols' wardrobe. In fact what you wear is only part of rebellion, and this only includes not wearing main stream brands because "your supposed to." A true rebel wears what they feel, not what they are told.
Show people how little you care by ignoring, and just plain defying the rules. Try just flat out leaving and ditching thing you don't want to do (like school). Small things like that help you get comfortable rebelling and getting the right mindset.
Do what you want to do. Not what other people want to, but you.
Just because you are a rebel doesn't mean you have to be a jerk. Tons of rebels (including goth and punk kids) hang out with the normal, nice people of the world.
Remember that getting arrested, can ruin your life. BE SMART. Something like stealing something as small as a Candy Bar, or wallet, can get you in BIG trouble. And chances are, you won't want to be a rebel forever. Chances are, you'll grow up, look back and feel really regretful about your decisions in the past.

[edit] Warnings

Don't force yourself to be a rebel because it's cool. If you don't enjoy defying things and you think you feel like you'd rather live inside the lines, do that. If you force yourself to be a rebel, things could go wrong, and you could be tagged a poser immediately.
Don't get too out of control unless you really want to.
Always be prepared for the consequences of this lifestyle. People will try to stereotype you for being how you want to be.
You don't need to prove yourself to anyone. Don't do something stupid just because your friends are doing it.