Well, since you didn't want to give your age, I'm going to assume that you are young. Teenager I'm guessing.. So here's what I think..

You're young, and experiencing a very typical self-confidence problem that almost all teenagers go through.. It's nothing to worry about, and with age, trial and error, and socializing, you'll gain confidence. Just set yourself some easy goals, and achieve them. Don't set yourself up for failure. Don't try to date the hottest chick at school. Start simple, and work your way up. Confidence comes from achieving set goals. Start off with something you KNOW you can achieve if you really want to. And once you achieve that goal, you will find that taking on something a little more difficult, will be a much easier task. If you've never kissed a girl, then set a goal to kiss a girl.. Not a popular girl, or a really hot one.. Just any girl will do.. Of course, you aren't going to brag to your friends about it, but it will boost your confidence. Simply because you will no longer be a complete "noob" to kissing... You'll have a much better idea of what's involved, and will be ready to take on something more challenging.. A better looking girl, more popular, whatever.. Just don't put on shoes that are too big for you.. Take steps that you know you can make..
Also, just remember, your not the only one with this self confidence problem.. Just about everyone that you know around your age is dealing with the same thing. When you look at your friends, you most likely won't see any signs that they are dealing with the same problem as you. They probably seem OVER confident if anything... But, stop and think for a second. When you are around your friends, or people your age, do you let them see any signs of your low self-confidence level? Of course not.. You hide it.. You pretend that you are confident. And your friends do the same thing. The more confident your buddies appear to be, the more likely it is that they are even less confident in themselves than you are with yourself.

I'm only 24.. My teenage years weren't that long ago..

And by the way, even if you're older, or younger than I assumed, the basics of my responce remain the same. Set goals you know you can achieve, and follow through.. Confidence will follow success, no matter how small the task may seem.