I've read in several books that its OK to start plain whole milk yogurt at 6 months. Another source (including my ped) suggested to start at 9 months. Either way, you're over the hump.
Suggestions for additions to plain yogurt:
Pureed or mashed fruits including apples, pineapple, papaya, peaches, plums, pears, apricots, banana, melon, tahini and finely minced parsley, natural nut butters other than peanut (like almond or cashew) if you don't have nut allergies in your familiy history. I also add a little cinnamon or ginger powder to yogurt with the appropriate fruits. Try mixing pumpkin puree as well, with ginger and cinnamon (tastes like tangy pumpkin pie).
I also add ground flaxseed to yogurt along with whatever fruit.
Use your imagination. Its so much better than the added sugars.
At about 1 year try berries. Strawberries are a potential allergen so watch closely when you try them.