There is a popular axiom that applies to the pursuit of confidence, which is that you should fake it until you make it. Part of being confident is being perceived as confident. When people think of you as confident they treat you differently, and this treatment, in turn, reinforces the idea that you are a confident person. So even if you do not feel confident, you should fake being confident in the actions you take, and that will, in turn, make you more confident.
One important way in which to appear more confident when taking action is to increase the amount of eye contact you make with other people. Eye contact is a sure sign of a confident person, and a person who looks away, and especially down at the ground is not, generally, a confident person.
Taking action is about not being shy, and about saying and doing what you want, instead of deferring to other, more confident individuals. To that end, you should endeavor to speak your mind, interact with people and act, rather than thinking about acting. Even if what you say or do ends up being wrong or embarrassing, just shrug it off and keep taking action.
Another important thing that confident people do is smile. In fact, just about any facial expression is important, and relates back to expressing yourself. Smiling is especially important, however, because it signals to other people that you are confident about who you are and that you like them or what they have to say. Remember that just about everyone else has doubts about their own self-confidence sometimes, too, and by smiling at them, and making them feel good about themselves, they will naturally look more positively at you.