I actually agree with Steve on something for a change. :-) While I see Philip's point that you should talk to someone you know, I don't think it hurts to comment, though we can't become your counselor.
You're the only one who can stop lying - no one can do that for you. You might spend some time thinking about how it has affected your relationship. While lying might seem better or prevent a problem in the short-run, it's always worse in the long-run. I feel it also shows lack of respect for the person you're lying to.
Since you apparently have not figured out how to stop on your own, I could assume this has become habitual and that maybe you cannot help it, in which case you do need to get help from somewhere. It might be embarrassing to ask for help from your parents, but if you admit the problem and the need for help, they will appreciate both your honesty about it, and your wish to solve the problem. But you must be serious about it.