Who can you tell? That seems to be a really important part of your question, right? I think that, because of your embarrassment and fear, you should tell a priest in confession or a Catholic counselor of the same sex as yourself, in a private setting with a promise of confidentiality.
One of the things you are likely to hear is that it is hard to stop. I'll tell you that now so you can say you've already heard it. I want to quickly add that your desire to stop is already a pretty good guarantee that you aren't heading straight to hell. People on the way to hell aren't trying very hard to go in the other direction!
The reasons you want to talk to somebody are these:
1. Knowing why you masturbate is a good step toward figuring out how to quit.
2. "Dumping" your secret will help you find a way out as well.
3. Someone else can help you figure out what ELSE to do, when masturbation seems like the only possible way to relieve built-up tension, sexual or other.
In case you take awhile to find that someone, I would like to suggest that you increase your physical exercise. When you're tired physically, you fall asleep more easily--and that will help you resist the temptation to masturbate. Secondly, pray over your choices of media. Some magazines, books, movies, cable-TV shows and Internet sites are practically engraved invitations to get horny--and there you are, all alone!
3. Don't dwell on it--in either a dreamy, pleasurable sense, or a "I'm headed for Hell" condemning sense. Replace thoughts of sexual gratification with thoughts of doing good for other people. One of the least desirable aspects of masturbation is how self-centered it is--the very opposite of the Christian call. It's not the sex part so much, as the selfishness part. Work on being generous and I promise you won't be headed to hell.