I used to have this problem when I was your age (or just a bit younger).
I certainly don't have it today, but it's funny cos I really can't put my finger one any one thing that made the difference.
If I had to guess I'd say a big part of the reason was simply a greater understanding of the world around me and of the ways different types of people behave. It's like the more you understand about life and about people in general the easier it is to have conversations with them.
It's also a matter of general self-confidence. The better you feel about yourself the less you will care about what other people think about what you say. This definitely helps because as you're learning it's a given that you're going to say some really dumb things. Hell, I still say some very dumb things today (but way less than in the past). Internally I cringe a little (especially at some of the stupider things I occasionally blurt out) but I don't let it show.
A key point is that other people don't know what you're feeling like on the inside. They can only guess and their guess is going to be based on how you appear/act on the outside, not on how you're feeling on the inside. So if you act as though you're unaffected more often than not they will treat you as though you're unaffected, and this usually works to your benefit. (I mean, if you say something dumb and then you sit there like you KNOW you said something dumb, that will definitely work against you. People will write you off and not care about anything else you have to say.)
Think of all the times someone else says something really stupid. If they don't look like they're too affected by it -- in other words if they appear confident -- you might think to yourself "what a dikkhead" but you probably don't completely write them off, right? But even "cool guys" can fkk up every now and then so most of the time it barely affects them. Now, if it's someone who CONSTANTLY says dumb things, well, in that case they are usually shown the door or ignored fairly quickly. Don't be that kind of person.
In the long run, I think, if you're prepared to practise and commit yourself to getting better then in time you will. You just have to be prepared to put yourself out there and occasionally feel uncomfortable. It's all part of the learning process. It may seem like "too much work," but I think it's worth it, because the ability to hold your own in a group conversation definitely reflects well on you if there are chicks around.
And one more tip, don't think that you ALWAYS have to have something to say, especially if the group of people isn't to your liking. This is a mistake I made when I was learning. I suppose it's part of the learning curve too, but the sooner you realize that some groups will be more to your liking than others the sooner you will realize it's a much better use of your time to concentrate your efforts on the groups of people you like rather than on groups of people you don't like. If you genuinely don't like a group of people, don't feel bad at all about ejecting from that group or just keeping quiet (eg if some of your friends want to talk with those people for whatever reason).
And what the hell, one more tip. If you're REALLY, REALLY shy, don't start off by trying to tell stories. People react not only to WHAT you're saying but also to HOW you're saying. If you're shy and tentative, people simply won't care about your story. A better way would be to first get used to talking by making shorter comments. Personally, with other guys you really can't go wrong with making compliments on other peoples stories. Don't do it like some azz-kisser -- that will work against you mostly. But if someone tells an impressive story it's perfectly fine to be impressed by it and let them know you're impressed (in a "manly," confident way). Save it for people and stories (in that order) you're genuinely impressed by. If you do it with people you don't care about or you think are "lower" than you or with stories that aren't really all that impressive, it won't have the same effect on your learning process.
