It really depends.
If I just picked up a lot of books from the library, I'll read those first because I only have a short amount of time to finish them.
If it's for Virginia Reader's Choice, I'll generally read those before anything else (because I really want to try to read all that I can before our meeting in October).
If it's a VRC from the library, then that goes straight to the top!
If it's a book in a series, then I try to read them in order before I start anything else. I don't want to forget anything from one book to the next (so I'm soooooooooooo screwed when the next Game of Thrones comes out! I'm gonna have to read all of them all over again. Ok, that's not too much of a hardship!)
If it is none of those, then I put them on a shelf in the general order that I'd like to read them and going from right to left, I'll just take the next one off the shelf.