Walk into a bar and punch the biggest, baddest looking guy in there. You'll learn how to be confident real quick...
Just kidding. Don't do that!
There are 4 very simple things you can do to become a more confident person, and all of them will benefit you in other aspects of your life.
1. Learn. You know all of those well-spoken guys you hear talking on TV and in newspapers? One of the main reasons they are so confident when they talk is that they have spent a lot of time learning. If you ever get the chance to go back to school, do it. Or, if you can't go back to school, read more. When you read, read aloud so that you hear what it sounds like to improve your own vocabulary. Intelligent people are usually some of the most confident people. Look at Obama. Even if you don't like his politics, you can't deny he is a very intelligent, charismatic, confident guy. If you talked to me in person, you would find that I carry myself almost the same way he does, just without the money, politics and fancy suits.
2. Explain to other people why you're proud to be who you are. Nothing says "this guy is confident" more than someone who is willing to openly discuss why they are the way they are. People who lack confidence will avoid that question/topic entirely. When someone asks you why you chose to shave your head, tell them the honest truth. Tell them how it makes you feel to have your head shaved, what it means to you, the advantages/benefits of shaving your head etc... You can always practice this in front of a mirror at first without any outside pressure.
3. Fix your posture/walking style. If you are one of those people who walk with slumped over shoulders, straighten up! Also, if you can help it, don't walk in a slow, lethargic manner. If you've ever heard the phrase "walk with a purpose", then try to visualize in your mind what that would look like, and then try it. You will be amazed how much your posture and the style of your walk affects your confidence. People are much more likely to take you seriously if you walk with your shoulders back, chin up, and at a nice, brisk pace.
4. Compliment other people. Many times, when we are feeling insecure/lacking confidence, we take it out on others in the form of insults, bad attitudes, and just bad communication in general. If you can break into the habit of complimenting other people, they will return the favor, and you'll build more confidence that way. It takes confidence to compliment others, because confident people are already fine with the way they are. They don't need to be told they look great since they already know it. It's like allowing your mindset to create the essence of yourself.
I hope this is helpful for some of you. There are many more ways to become confident, but these are the main things I have done to help me, and I've had great success.