You may only encounter snow in the high country, and not much at that depending on where you go. In the Valley in summer, it is hot. Not Florida hot with high humidity, but hot. Check the park's website for weather info.

I highly recommend the Rough Guide to Yosemite. Amazon or your local bookstore will have it or can order it for you. Retail price, $9.95. Also Tom Harrison has a great map of Yosemite.
There are many other guidebooks, plus info on the park's own website. The concessionaire also has its own website to make camping reservations, which you will need for some camps. Backcountry requires a free backcountry permit, which you can get once you get there.
Don't get me started, you know how I get.