Does Line 6 still make the USB port and is it Mac compatible?
They have things like the
TonePort that pair some digital effects processing and amp modeling with digitizing.
M-Audio, Edirol/Roland, and a bunch of other companies also make more straightforward digitizers / external audio cards. Pricing is generally between about $70-150 for most of them.
Since the OP is of few words...
The big question for the OP is... are you trying to record only, or record
and amplify?
If you convert from 1/4" TRS to 1/8" TRS (like the cable Jessica linked) and plug it from your guitar straight into the mic port of the computer, you won't get very usable results because your guitar doesn't output line level. The cable is actually designed, AFAIK, for use with the Griffin iMic or whatever it's called, which
has a preamp in it.
If, however, you convert from 1/4" TRS to 1/8" TRS and plug the cable from the
amplifier's line out to the computer, the results will not be too bad. As others said, it won't be studio quality -- it will have some noise. But it will be fairly usable for garageband type projects.
So basically, if you have an amp, you can just do this with the 1/4" to 1/8" converter. If you don't have an amp, then you really should buy another piece of hardware -- either an amp or something like the toneport.