If you want to sweeten with sugar, after fermentation is complete add 5 crushed campden tablets and 1 1/2 tsp of potassium sorbate. Stir well with a sanitized spoon and leave for ~24 hours before adding the sugar. Add the sugar mixture and leave ~3 days to make sure fermentation doesn't resume. If you stabilize like this you'll have to force carbonate of course since the yeast will be out of commission.
Alternatively you can just go with Ed's suggestion and add 7up or Sprite when serving.
Alternatively you can just go with Ed's suggestion and add 7up or Sprite when serving.