2 things:

Firstly if you fix your self image and confidence your body language will follow. If you genuinely believe you are the dogs danglies then you will walk and talk like you are.

Secondly posture. Focus on getting your posture correct i.e. no slouching, shoulders back etc. if you google it I am sure there are plenty of posture adjustment webpages and videos.

but if you have the first one sorted then kino and everything else will fall into place...