Don't go cold turkey! I was addicted to cola for over 10 years. I seriousely drank nothing but cola every day... About two bottles a day. Once I realised it was a problem I tried several times to go cold turkey off it and had some horrible side effects - sickness, shakes, headaches, runs, irritability, dehydration... It was awful and made me very sick. I managed to come off it in the end by slowly weaning myself off it a little bit at a time.
Sadly it's left me with awful teeth decay (which has all been fixed now, but I still feel ugly) and kidney problems...
I didn't drink the diet stuff though as I got shakey if I tried the diet stuff.
Good luck! You'll feel much better once you kick to coke!