As always, feel free to e-mail your questions to me or ask in the forums . We have an iMac G3 with Mac OS 8.5. Anyway, this process should be fairly simple.
Lets start with the hardware required.
Essentially we want to connect your turntable to the sound-in port on your iMac. So we need to get your turntable up to line level. If you have a stereo receiver, it will perform this function for you. If you DON'T have a receiver with which to accomplish this goal, you can also get a signal amplifier at Radio Shack as well. On the other end of this, you'll need a CD burner.