Does that include melted cheese?
After slicing and toasting mine often does.
Anyway Sourdough Starter:
Actually pretty simple, you're trying to cultivate wild yeast. You can use all-purpose flour for this, it's the cheapest. However you'll get different yeasts from different flours, f'rex if you use graham flour your bread will wind up much sourer. Once your starter is going you'll get the best results from using the same kind of flour you bake bread with.
Take a jar and put a scant cup of flour and half a cup of lukewarm water in it. A smooth-sided glass jar works best, like an old peanut butter jar, so you can see the yeast bubbling inside. Water temperature is important, max-hot tap water is enough to kill yeast and cold water will make your yeast multiply very slowly, so try to get roughly body-temperature water. Also, don't use chlorinated tap water, it will kill the yeast so if your water has chlorine in it, let it sit so the chlorine evaporates out before using it. Mix the starter thoroughly so there's no dry flour anywhere. Cover it loosely so the CO2 the yeast makes as it eats the flour can escape and leave it all day. A slightly warm location like on top of the refrigerator helps.
The next day throw away all but half a cup. Add another cup of flour and half a cup of water and mix it up again. You may see some tiny bubbles at this point but not always.
By day three you should see small bubbles appearing in the mix. It may start getting a sour yeasty smell. Repeat the entire routine, throwing all but half a cup away and adding another cup of flour and half a cup of water.
Repeat this for a week. By day 5 it should be pretty bubbly. By the end of the week, it will be ready to make bread with.
Continue feeding the sourdough starter daily forever. It will continue to subtly change and become more effective over time, and your breads will gain a better and richer flavor the older the starter is. Certain Amish families are known to have starters over three centuries old, passed down in the family for generations, that create incredible breads. Many people name their starters and treat them like pets.
If a thin liquid forms on top of the starter, you can stir it back in. This is called "Hooch" and it's mostly alcohol, it indicates the starter isn't being fed often enough. If you don't want to feed the starter every day you can put it in the refrigerator, this will retard the growth of the yeast enough that it only needs feeding about once a week, however it will need to be fed at room temperature for a day or two before it regains enough strength to make bread.
Protip: When starting out, put a rubber band around your jar where the top of the starter is and you'll be able to see that it's actually growing. Once it's really going you probably won't need that visual clue that it's growing anymore though...