Sourdough starter is yeast and bacteria in a paste of flour and water. There isn't any single recipe, there are many versions. For purists, it has to contain NO COMMERCIAL YEAST. Fresh flour has naturally occurring yeast in it, always remember to start a batch of sourdough starter with fresh flour. Between this and the wild yeast that floats in the atmosphere, you should have plenty of yeast for your starter.
The simplest recipe is to add one cup warm water to one cup flour in a clean, sterilized, wide-mouthed jar, like a pickle or mayonnaise jar. Stir. Let it set uncovered or lightly covered with a single layer of cheesecloth (so it can catch wild yeast) in a warm place (between 70F-80F). Every 24 hours, throw away 1/2 of your starter and replace with 1/2 cup warm water and 1/2 cup flour. If you have a lot of natural yeast in your local air, in three or four days, you'll find your starter beginning to froth and get a yeasty or beery smell. It might take a week or ten days if you're not lucky.
If it takes longer than 10-14 days, or if it begins to smell rotten, not just sour, or if it develops mold or a black or brown color rather than a white, creamy, light tan or pale yellow color, throw it all away, clean and sterilize your container (it might have picked up some microorganism that kills yeast) and start over. Or you can buy a commercial starter or add commercial yeast to your starter mix. Sourdough purists will cringe, and the snobs among them will look down upon you, but you'll be able to make bread. If you wish to purchase sourdough starter, Google “buy sourdough starter” and you’ll find many choices.
When your starter bubbles a lot (not just at the top) and begins to get spongy (a kind of "pouffy" look), you are ready to refrigerate and/or use.
Keep your starter in the refrigerator, make sure that you have at least one inch of headroom in your jar and leave the lid slightly loosened so gas can escape. Feed once a week, discarding half and replacing with 1/2 cup water and 1/2 cup flour. Sometimes you'll get a layer of watery liquid floating on the top of your starter. This is called "hooch." It contains alcohol and may smell like beer, but it isn't beer. Don't drink it, just pour it off. (Or stir it back in if you're going to be using the starter soon.) If this makes your starter a little dry, just add a bit more liquid when you feed it next. Try to keep the consistency of your starter, well, constant, adding more flour or water as necessary.
I am assuming that you have some sourdough recipes, so I will only add, "Good Luck."
Oh, one last thing. Wild yeast is just that, wild. The yeast that lives at your house is different than at my house. It may even be different than that of the guy across town. Unless you live in San Francisco, you're bread will likely not taste exactly like San Francisco sourdough bread. It will be distinctive to your area. Have fun with that with the snobs. Look down your nose. Give a mildly offended look. "San Francisco sourdough? Certainly not. THIS is OKLAHOMA CITY (or wherever) sourdough."