Ok, here's my deal... I bought a car, the guy didn't have the title... but a bill of sale was written out and signed by both parties... can I use JUST my bill of sale to register my car? Or do I NEED the title? And if I do need the title, how do I get one?

PS This is in California

Unless it is a antique car that did not have a title, you do not own the car unless you received a signed title. You don't even know they own the car, you don't know that there is not a lien on the car or anything.
You need to run the VIN to first be sure it is not stolen, but to see who actually owns the car.

If you don't get a title, you don't own the car yet.