I recently quit Coke/Pepsi as well (for the bazillionth time - hopefully it sticks this time). I didn't get headaches, but I got extreme fatigue from the lack of added caffeine boost. Drink lots of water, and just wait for it to go away. It will in a few days, trust me. If you REALLY can't stand them, though, I recommend tapering off, as some others have suggested.
For me, caffeine is like alcohol is to alcoholics. I can't have a glass of Coke, even to celebrate one night. Probably never again, because I have one glass, and I go back to drinking several glasses a day, every day. And I was suffering severe health side-effects. It's terrible. =/
I wish you luck!