What you'll need is a mixer (a DJ mixer is fine), hook your turntable up to it, then hook up your mixer to your soundcard. The mixer has RCA outputs, and the soundcard has 1/8" inputs, so you'll have to buy a wire with those specifications. You can find them all over, and they only cost a few bucks.
If your amplifier has an output channel, then you can also hook up your turntable to your amplifier and hook up the output channel to your pc's input. I don't know all the cable/output/input specifications...look at the channels to see which kind.