It depends on what you want to shoot. 'Freelance' doesn't tell us anything...that just means that you don't work for a company in particular.
It helps to shoot what you like and what you know. So if you are into a sport, then shoot that. If you like people, then shoot that. Build up a portfolio and go from there.
There are any number of ways to make money with photography...but you have to find a market for your shots. Keeping in mind that you are 15 and probably don't have a lot of experience...you will need to find people that won't mind about your age. Shooting weddings, for example, might be a tough sell. Shooting portraits of kids your age (for their parents) might be a better idea. Also, consider that these days...most teens are probably active on the internet...MSN, Facebook etc...all that stuff. They usually have avatars or some sort of photos of themselves...but they are usually pretty poor photos. You could go around offering to take better quality photos for them to use on their on-line communities. Charge them $20 a piece, or something like that.