First of all, what are you making ? Secondly, is bicarbonate of soda one of the other ingredients ? If yes, then leave BOTH out and use baking powder instead (assuming you have some of that and that you are making a cake or scones). Baking powder is usually a mix of bicarbonate of soda (aka baking soda) and cream of tartar.
Cream of Tartar is often used to add acidic ingredients to baking powder but is also used to help in producing the nice, soft peaks to beaten egg whites for meringues. If the recipe is calling for cream of tartar to be added to egg whites, one can substitute plain white vinegar equal to the same amount of tartar required...