Be aware that some locales will not start a starter, especially at some times of year. While at least some wild yeast spores are nearly everywhere all the time, they're isn't necessarily enough of the desirable sorts.
In my experience, no matter what you use to start the starter will eventually be taken over by local spores. It may take weeks or it may take decades, but it's always seems to happen -- at least to me.
Fortunately, you can buy or otherwise obtain a starter culture to start a starter anytime. If you want a sure-fire starter of the highest quality, you can get one for free (plus an SASE) from "Friends of Carl." Carl Griffith Sourdough Page
It's not the word's best website, but it's one of the world's best groups with one of the world's sweetest sour missions. Visit them. You could learn something.
Usually you can get a starter started by starting a poolish -- just keep breaking it in half, feeding half, and using the other half. You'll taste it growing increasingly sour as wild spores begin to take over the colony. Usually takes about two weeks to go from poolish to sourdough starter -- which oddly enough is about the same time it takes a starter to ripen to the point of usability.
If you cover the starter's vessel tightly, you run the risk (probability) of something a lot more interesting than mere infection.
My starter's home has a a few, wee holes in the roof.
Starters are like keeping wild animals as pets. It helps to have a sense of humor, and it's good to bear in mind that the relationship might not be permanent.
Most people keep their starters pretty darn slack. That's why they're starters and not bigas. In turn this makes for a very slack dough. The trick to kneading a slack dough like sourdough, or ciabatta dough, or just about anything similar, without using a ton of bench flour that will upset your ratio, is to repeat the "autolysis/french-fold" process two or three (count them, 2 or 3) times, at ten minute intervals, before kneading. It doesn't dry out the dough, but does stiffent it and make it a lot less sticky. The knead to "windowpane" will go a lot faster, too.
Hope this helps,
PS. I just learned that a few months ago watching a Peter Reinhart video; and will adjust my olive bread and onion-dill bread "technique" sections accordingly -- if you're baking either, get in touch.
PPS. That's Reinhart for you. Always coming up with something to make it take more time.
PPPS. Finally, if you don't get what I'm talking about with autolysis/French-fold at all, PM, email me, or post it here, and I'll start a thread on it on grounds that if one person asks, a baker's dozen want to know.