A starter, which is a mix of active wild yeasts and bacteria, is the key to leavening all kinds of sourdough-based delicacies. This recipe, adapted from Artisan Bryan, calls for a combination of flours; the rye flour helps speed up the process. Once the starter is established, you can continue to feed with a mix, or transition to entirely white flour. Note that you don't add any yeast yourself; instead, you allow the wild yeast present in the flour to develop over time.
To ensure your starter is ready to bake with, always feed it a day before you plan to mix your levain. If you are very confident it's lively enough—it has reliably doubled in volume after feeding and is stretchy and webby—you can replace the levain in bread recipes with fed starter equal in weight to the parts of the levain combined.