I wanted to install a new pickup in my GAX. It has a wire that looks like that of a P-90, but there are two wires inside thing that goes to ground. Hope you know what I mean. I'd give some pictures

This is how you wire a P90: http://www.seymourduncan.com/images/support/schematics/p90.jpg

This is how the wire of my humbucker looks, with metal shield and two wires within.

So I thought wire the metal shield to ground and the inner white to the volumepot, but what to do with the black one?

Help!? How do I wire this?
I'm guessing that the extra wire is a coil tap but it could just be an extra ground wire. You need to get a multimeter and test the DC resistance between the white and ground and then the black and ground. Whatever measures the highest DC resistance will be your hot. If the extra wire reads a slightly lower resistance but still has a resistance of at least a couple K then it's a tap. If the extra wire has a very very low resistance or no resistance then it's an extra ground wire and should be soldered to ground. If, when testing, the your meter reads infinite resistance then it means you have a wire to ground the coil and a separate wire to ground the bassplate.
Not taking any online orders.