1. You have to truly want it. I've come across many people who complain about how their low self-esteem is blocking them from achieving what they want, yet when others try to help them they don't open up. Well, you can't do it if you don't open up. So, unless you're ready to do that you're not ready to change your attitude. It also can't be achieved overnight; it takes time and hard work. Also, know the line between low self-esteem and achievement. An overall low self-esteem does not ensure non-achievement. Look at Johnny Depp - he has social anxiety but is living the celeb life because that's how much he wants to act. So, stop letting your timidity define you or determine your life - work around it. Cheat.
2. Start from your passions. If you want to be confident you need a place where you can "train" yourself. The best place is your passions and hobbies. As a little girl I was so shy, I couldn't even talk to my teachers when they spoke directly to me. I would just freeze up in fear like an ice cube. But the summer before high school I suddenly got the urge to change my life. I always had a strong will to sing in front of people. So I auditioned for a summer training program and passed. When I got back to school from the program it was my first year of high school. My personality had changed already. That's because at the summer program I realized I was good at singing, and that gave me confidence in other areas of life - my looks, studies, dating, everything. This is because once you strengthen your passions you'll have a stronger, more solid identity and see yourself as a worthy person. It's called amour propre. You'll no longer see yourself as "just some guy/girl" and start seeing yourself as a unique individual who wants to be recognized for his/her talent(s).
3. Surround yourself with supportive people. Anything in life is much easier to achieve when you have external support. Have people around you who will cheer you on, protect you, and guide you through your journey toward your new, confident life. It's not easy to run a race when you don't have anyone there cheering for you - it's the same in this case. You need love. It'll also be advantageous to have a teacher/trainer to guide you through developing your hobby/hobbies. On that note, here are exactly the people you need (at least one): a close friend who knows and cares about both your insecurities and your talents; an authoritative figure, i.e. family, a teacher/teachers at school, a guidance counselor, that nice random lady you chat with every morning at the bus stop.
4) Get rid of toxic people. They will only distract you from achieving your goals. If you can't simply cut them off, perhaps because they're a family member or two, learn to at least become deaf to their negativity - selective hearing! You don't need anyone's approval. This is completely about you, not them. The thing about haters is that they may seem to know more than you but in fact don't - clearly, because they don't see or care about your strengths! People can't completely know other people. You know you the best. Nobody else knows shit.
5) Learn to accept and toss what haters, or potential haters, think of you. This relates to #4. It's not possible to completely ignore them because we're human - social creatures - after all. We will always care and as a result always have insecurities. So the key is not to really ignore negativity but to accept it, and then toss it all in the trash behind you while you rise to the top. And while you're at the top, enjoy the view of them standing down there with dumb, hopeless looks on their faces. It took a while for me to learn to do this with some of my family members who have been unsupportive about several aspects of my life, including my music. But it's possible. Just go for it head first.
6) Lastly, believe in yourself. I know this is a cliché saying, but try to take it more seriously. Really think about what it means. If you believe you can do it, then you can do it (unless it's humanly impossible). Trust me, you have more strength in you than you think. You just have to learn to trust yourself first. Learn to trust that you yourself can guide you through the experience/change. And, actually, you're the only one who can. No matter how much help you seek, at the end of the day it's up to you. Your life is in your own hands. And this leads us back to #1.