First, you're going to need some sort of interface to get your guitar into the computer. In rare cases you can plug your guitar into an audio input jack on the computer using a simple adapter, but this is probably not a likely option. Instead, you'll want a USB interface like this one from Peavey
This gets the guitar signal into your computer, and then you use software to add tone & effects to your guitar. The Peavey X Port comes with amp-simulation software, but I've never used it... it was just one of the cheaper options. Another option is to get an effects unit that has a USB out, so that you can send a processed signal into your computer. These are more pricey.
Once you're able to play your guitar through your computer, you now need to be able to hear both the guitar, and the Jam Tracks audio at the same time. There are so many possibilities, I won't go into them, but simply tell you how my system works:
I have a Mac Book Pro, and use Line 6's "Guitar Port" which plugs into the computer via USB. I then plug the guitar into the Guitar Port, which has a nice big volume knob on it. My headphones are also plugged into the Guitar Port. Next, I had to set the preferences for the computer's audio out to go to the Guitar Port. I think I only had to do this once, and now it seems to do it whenever I plug the Guitar Port in. Now, if I play any audio on the computer, I hear it through the headphones plugged into the Guitar Port. I use the software that came with the Guitar Port to select effect patches for the guitar and can jam along... in your case I'd set the guitar sound, then go open the GT Jam Tracks player, hit play, and be able to jam along.
One nice feature with the Line 6 software is that it allows me to import a backing track (there's tons of free ones on-line) and then play that track right in the Line 6 software. It also allows me to balance the guitar and backing track volume, as well as the over-all volume.
Take a look on-line at the USB options and see what appeals to you. For what its worth, I practice with my electric three ways:
-through the computer with Line 6 Guitar Port
-through my BOSS DR880 which has built in guitar effects.
-using my Laney Lionheart amp and just having the backing track play through a stereo, or not use any backing tracks at all.
With the other two options, its hard sometimes to get a satisfying tone using the computer, but its still fun & useful!
