There are many great bread and baking sites online. One that I like a lot is Sourdough Home.

Interestingly enough, "sourdough" need not be sour. Indeed, many of the best breads that I know of are not sour at all and are made using sourdough methods, so the term "wild yeast" bread seems quite apt.

Here is a great link that talks about how to make a sourdough starter.

It is easy enough (and fascinating) to prepare your own starter. All kinds of methods are offered. It is recommended to start with coarsely ground rye flour, as rye is said to have lots of the organisms that you want to start a sourdough culture. Some people purchase sourdough cultures (King Arthur will sell you a living culture in a plastic jar). Some people just start with flour and water and rely on picking up the yeast and bacteria from the air (or the flour they start with). Some add a bit of yeast, though this is frowned upon by purists.

I am told that certain bacteria can interfere with the successful startup of a proper culture. These are leuconostoc bacteria (and are used in the fermentation of cabbage to produce sauerkraut). In particular, Leuconostoc citreum is a known troublemaker and a gas producer that gives a false impression of success, only to stall 3 days later or so. The recommended trick is to use unsweetened pineapple juice during the first couple of days when making a starter. See this article by Peter Reinhart. Apparently the pineapple juice establishes a pH level that discourages the leuconostoc bacteria, but is favoriable for the yeasts that are desireable in a sourdough starter. There is more about this here. as well as here in a pair of fine articles by Debra Wink. About 1/2 teaspoon of ascorbic acid works well, but many vitamin C pills are buffered and won't yield the pH control that is desired, and finding a source for unbuffered ascorbic acid is problematic. Other bacteria (Aerococcus viridans) do not produce gas, but sometimes proliferate in the early days of starter production. Enterobacter cloacae produce unpleasant smells.

Along with pineapple juice, it is recommended that you stir your starter 2 or 3 times each day for a minute or so. During the "seed" stage this can help avoid bad organisms getting a foothold. The yeasts and desirable lactobacillus are happy with the extra oxygen and the bad bacteria (often on the surface) and especially molds get stirred into the acidic environment and are deactivated.

Another writer suggests that when you get a good starter going, you hedge your bets by freezing some flakes of it in zip-lock bags so that if your starter goes bad or gets invaded by unhappy organisms, you have a "seed" culture to start over with.

There are 4 phases in the development of a sourdough starter from scratch. Note that the initial bacterial stages pave the way for yeasts later.

1. Phase 1 - no visible signs (day 1 or so).
2. Phase 2 - lactic acid bacteria, but few bubbles if pH is made acidic as by pineapple juice.
3. Phase 3 - very tart taste, gluten disappears, noticable tiny bubbles.
4. Phase 4 - yeast growth, lots of bubbles, yeasty smell.

Debra Wink's recipe

Day 1 mix:
2 tablespoons whole grain flour (wheat or rye)
2 tablespoons pineapple juice.

Day 2 add:
2 tablespoons whole grain flour (wheat or rye)
2 tablespoons pineapple juice.

Day 3 add:
2 tablespoons whole grain flour (wheat or rye)
2 tablespoons pineapple juice.

Day 4 (and daily until yeasty) mix: 2 ounces (1/4 cup) of starter (after stirring down, discard the rest).
1 ounce flour (any kind, scant 1/4 cup)
1 ounce water (2 tablespoons)