The amount of sugar to add to a wine can depend on the variety and largely on personal taste. Adding before fermentation gives higher alcohol, adding with Campden tablets at bottling time adds sweetness to the flavor profile
A great resource for winemaking can be found at http://winemaking.jackkeller.net/
Jack Keller has a very large collection of recipes and I've found way too many wine recipes to try anytime soon. I currently have a strawberry, guava, orange, and Welch's red wine at various stages. All were from recipes on Jack's site.