Although I am (slightly) in my 30's, I haven't forgotten what it is like to be young... there's a lot of self-doubt that goes along with it, and frankly, adults in general only contribute to the problem.
Sure, you're young... sure, you've got a lot to learn... but at the same time, you also have a fresh perspective on things and have grown up with the Internet all around you. Those who are young are absolutely vital to success. Their ideas will be the ones that triumph in the end, not some tired, stale idea from middle America.
I know that many young people have heard this from adults before, but it's true... believe in yourself and follow your passion. This is a lesson that took me years to learn myself. When I was young, I didn't know what I wanted to do. This is, perhaps, fortunate, because today I can do most anything. Still, I look at a lot of young people on LJ and realize how incredibly smart and talented they are, and how much they could become.
I will tell you a few secrets that most adults won't tell you and many adults don't even know... Recognize your talents and don't be concerned if you are a bit cocky and arrogant about them. We all know that there are many things about the world that, for lack of a better word, just plain suck... you can do it better. Infact, if you really care about what you're doing and believe in your talents, hardly anyone in the world can do a better job than you.
I would also advise you not to limit yourself... nobody is "just a ______" (student, writer, secretary, programmer, etc.) I consider myself a writer, and a web designer, and an artist, and a poet, and a musician, and a promoter, and a leader, and an architect, and a businessman, and an evangelist, and a scientist, and whatever the f*ck I need to be at any moment to succeed. I don't concentrate on any one field in life... I am interested in everything. Because I care about and believe in what I do, I can do better than the best in any field on any given day. I may be arrogant to think this way, but it helps me to succeed, which is the most important thing most of the time.
...but not all the time. You don't succeed alone. You don't succeed by hurting the people who you're supposed to be helping. You've got to be arrogant AND humble... and the best way I find to be both is to be giving of yourself and your time and your talents, and recognize and respect other's talents too. If they care enough and if they believe, they just might be better than you on any given day.
Take this advice and run with it. Find a way to help LiveJournal that you care about. Find a need and fill it. Let me know what you want to do, of course, but feel free to show initiative. If you have good ideas, I'll do everything I can to help you implement them. Above all, though... believe in yourself.