Confidence is all you need more of - in yourself and what you have to offer.
I wouldn't even focus on trying to be 'alpha' if that's not really you. Some people are BORN alpha and don't feel comfortable unless they're jossling to be a 'leader' of a pack and if you're not one of them, don't even go there.
Concentrate on confidence. You will be happy to know it grows with age.
Experience more. Do more. Face your fears.
The smallest things can do wonders for your confidence
E.G. If you fear swimming because you lack confidence or think you look out of shape or whatever, go to the public swimming baths.. have a swim.. get showered and walk out of their feeling like a million bucks for having the balls to present your body in public.
E.G. If you're not comfortable talking to women, go to the bakers, cafe's, shops etc and just make small-talk with people as you're buying things. Comment on the weather.. hell.. go for a coffee in an average coffee shop and tell them how much you prefer their coffee to starbucks and crack a little joke..... they'l probably say something back and you will feel GREAT...have your coffee and walk out of their feeling like a million bucks.
Talking more to people will become an addiction and before you know it, you will begin to really enjoy socialising and feel more confident coming out of your shell and expressing your thoughts more etc.
Constant interactions like these will improve your confidence like nothing else over time.. and things do take time.
Hope this helps,
The Muscly Jerk