Do you want to know how to act around the girl you have a crush on? I have some tips to get that girl for sure.

[edit] Steps

If she is at your house greet her with a :"Want to come to my room to play Guitar Hero", now thats just an example. When you ask her to go to your room don't be perv, ask her to do something she will like while in your room.
While in your room you guys should be playing a board game or a video game. Let her win the game and tell her she is really good, girls love compliments.
After playing your game you should watch TV or a movie. When watching a movie make it a scary one, girls will be wanting to cuddle. If you watch a romantic, try to act a little sad, maybe even a little tear because girls love sensitive guys, they find that brave.
Look at her but not in a weird psycho way. Look at her here and there. If she looks at you back, give her a little smile, then look at the TV. Girls love sweet, flirty guys.

[edit] Tips

Always smile, but not in a psycho way, in a sweet way.
Compliment her on the little things that she probably worked an hour on trying to impress you with.
Be brave. Stick up for her when people are being rude to her. She will think your Superman saving the day.
Tell her how you feel about her. If she smiles she feels the same way, if she makes a confused look she probably likes you too she just didn't know you felt the same way, and if she looks sad she likes you as a friend and thats it.
Have manners.
Be kind to everyone even your little siblings.
Give her little pokes here and there.

Don't be a jerk, that is a major turn off.
If she doesn't act nice to you after you tell her you like her, shes not worth your time.
Don't change for a girl, be yourself.
Don't act mean to her to get her attention because she will think your a jerk and go for a nicer guy.