Hi Dani,
You'll likely not be able to reserve a space inside the park by now, but try anyway. Go to the Yosemite National Park website, and there will be a link there for reservations.
There are many great camps just outside the park as well, and you'll probably have better luck getting a spot in one of them. There are shuttle busses running in and out of the park all day long,( the preferred "ECO" friendly way of touring the park) so being outside is generally not too much of a handicap.
We like this guide: http://www.twenga.co.uk/book/foghorn-outdoors-california-camping-the_4287389.html
Get one used and the only thing the older versions will be wrong on is the current pricing, but you'll get an idea for comparison.
I've gotten decent tents from walmart, if you are only gonna use it a few times. There are also great deals to be had at local thrift stores, for example if you are flying into LAX, here are some from Google.com: (THRIFT STORES LAX) http://maps.google.com/maps?f=q&source=s_q&hl=en&geocode=&q=thrift+store+lax,+CA&sll=35.347266,-119.00176&sspn=0.043755,0.077248&gl=us&ie=UTF8&ll=33.967281,-118.401375&spn=0.171412,0.30899&t=h&z=12
Before you leave, give it away.
Hope this helps,
Bill & Diane Staggs
In California with -snip-
[ Edit: Sorry, no promos please. ]