It is copper shielding tape. It helps block out interference from televisions, computers, etc. (Basically helps get rid of extra noise with an electric guitar.)
As far as learning to work on guitars, it has been trial and error. I did do a jewelry class in college so I had a little experience welding small things and using solder, but really it was just reading forums like this, looking at diagrams, and trying my best. Normally wiring for a Jazzmaster looks like this diagram below. You can see where each wire goes, so you just have to solder the wires where it says to. Other than that it is just making sure the metal parts are grounded correctly. That is also easy to test. If you touch a metal part of the guitar with it plugged in and there is noise that happens, then it is likely that part isn't grounded. If the quiet hum gets quieter, the part is grounded correctly. I fount a couple things on this guitar that was not grounded right.
When you modify wiring you can either ask people on forums to help and sometimes they will, or try things out and see if it works.