If you read some books on body language you will find that they are almost similar. The key is to look relaxed and confident.
You cannot control every single aspect of your body language all the time so the best is to focus on these things:
Posture: Stand straight, keep your head up, relax your shoulders and hands, keep your feet apart and don't be afraid to take the space which you need to feel comfortable(don't overdo it though

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Hands: Keep them relaxed and don't cross them in front of you. Don't gesture too much and touch all the people around you
Breathing: Breathe deeply and from your stomach(You can have perfect body language and still look nervous if you breathe fast and shallow). This also makes your voice more deep and powerful.
Eye Contact: Make strong eye contact and don't look down when a girl looks at you
Avoid fidgeting!!!
This is very basic. But it's really important to master it. After that you can(if you want) add more things to it.
If you want to improve your body language you can read books but I think it would be better if you look for articles in Google(they cover all the basic principles and are free). And also don't forget the movies. Actors have great body language(it's their main tool to affect the audience). You can see how Brad Pitt or Tom Cruise behave. Watch the movies Fight Club, Top Gun, Van Wilder, Hitch or any other like them. They are a great source of information on body language.