Is it bad to have a soda during pregnancy?

I am going to be 13wks prego and since finding out at 7wks I have not even thought about soda. Today we are ordering out at work and I am going to pick it up and have been thinking I'd love to have a fountain soda :-( but I know it isnt any good right? I have drank Ginger Ale for an upset stomach while prego but never normal soda. Am I being selfish by craving it and should I just get a water or is it ok to indulge once?