Hey guys,
Well, one of the wires came off of the jack in my guitar and i just need to solder it back on, everything else is cool with it.

But i need to go buy some solder and before I buy any I wanted to ask: Is there any specific type or thickness of solder that is best for soldering the wires in guitars?

Guitar is LIFE!!!!!!!!!!!!!
Any will do, just make sure its lead free otherwise the police will hunt you down, just kidding (apparently it's bad for the enviroment). for a small job like this using lead solder shouldn't be a problem.
1.2mm should do the job.