I think you're going to want to follow a recipe the first time you make wine- that'll help you decide what to add. If you're just asking about water, though, you add water to the level of wine you're making, or to dilute it if the SG is too high. You can also use some water if the ph is too low, and you think it's too fruity. Most fruit wines are best at 1.085-1.100, depending on what you're making. Some would be way too alcohol forward (like a strawberry wine) above 1.075 or so, though, so a trustworthy recipe is always good to start with.
