OK - what's you criteria? What time of year? How many people? There are several types of tent camping in Yosemite. There's reservable sites, walk-in (no reservation) sites, backcountry (backpacking), etc. The locations in Yosemite Valley are always going to be reserved well in advance during the peak summer months. The first-come, first-served sites vary depending on location, although most are well out of Yosemite Valley.
I'm taking a big leap here and am assuming that you're talking about tent camping near your car and near bathrooms and perhaps running water. A lot of people call it "frontcountry camping".
Here's where you need to start:
http://www.nps.gov/yose/planyourvisit/camping.htmIf you want to camp in Yosemite Valley in the summer, you'll need to do so well in advance if you want a reserved site. I reserved my site when reservations for certain dates (July 15-August 14) became available at 7 AM PST on March 15 - over 4 months in advance. They were pretty much all gone within about 7 minutes, although scattered single nights were available throughout the late afternoon. I didn't get my first choice and jumped immediately on my second choice. Occasionally someone cancels a reservation and you might be able to find a campsite. This is rare and more likely than no someone will beat you to it.
Bathing is also an issue. The only showers in the park open to campers are at Curry Village, Housekeeping Camp, or Tuolumne Meadows, and require a $5/person charge.
