we've talked about the many ways to make sourdough starter in past threads. I just started a new one, by using the skins from some plums that had a nice bluish-white 'bloom' on them. Grapes grow the same yeasts, too, wihch also looks like a 'bloom' of greyish stuff on the skin of a purple grape.
Just cut off some skins, put them into with some flour and a lot of water (we used rice flour due to a gluten-free dieter in the house, but wheat, rye, etc, all work great). After 24 hours or so, pick out the skins and feed the watery 'starter' just enough flour to give it the consistency of runny pancake batter, then incubate again. All this time you can leave it at room temperature. Within a couple of days it should grow a nice colony of tasty/bubbly yeast, the same yeast that came from the bloom on the fruit skin. After a day or a few days, the colony of yeasts in the starter should be robust enough that you'll see lots of bubbles in your 'batter'. At that point you save a little starter and set it aside/into the fridge to start the next batch next time you want to make bread. The rest of the 'batter'/sponge/starter that you just made gets used in bread recipes (I use a cup or two of 'starter' per loaf of bread and add enough flour to make the runny starter into a proper consistency dough. Sometimes I add oil, sometimes I don't, I always add salt, and I vary the flours to get different kinds of sourdough breads)

If you're lucky, the yeast you happened to capture might be a really robust leavening agent, and you may not need to add any storebought bakers' yeast to the recipe to get it to rise. If you don't like the flavor or if the stuff doens't seem to have a lot of 'oomph' as a leavening agent, try capturing another strain of yeast from a different batch of fruit. There are other ways to get sourdough culture, including just leaving a flour-water mixture out on the counter, open, till it ferments.