When I was 13-15 or so, I only drank soda. I would drink three, four sodas a day. It was horrible. Soda is the worst thing you can drink. Diet or caffeine free soda isn't any better (though caffeine free isn't addictive). Now I drink soda maybe two or three times a month, and then it's usually caffeine free.
Caffeine is addictive. When you try to stop drinking it after your body is used to it, you will go through withdrawal (not nearly as bad as alcohol/nicotine withdrawal, but still uncomfortable). If you really want to help your body and stop drinking, you'll have to tough it out for a while. Gradually would probably be best, so going down to one a day is good. Next go down to, say, half of a soda a day. Then none, and just tough out the headaches for a day or two (they don't last long).