18F4550 is the standard 40-pin USB-enabled PIC. there are also the 4455 and the 4450, which are the same except with less program and data memory, and are thus a bit cheaper.
I suggest you check out the microchip website, they have all their products listed, and you can sort them by categories. (for instance, list only the ones with USB)
And if you're just beginning, don't expect to be using USB any time very soon, it's more of a challenge than you probably realize. If you want to do it, get familiar with PICs in general (if you aren't already), get familiar with programming them in C (if you aren't already) and then go find a good book, article, or tutorial to get you started, and some solid example code.