Most turntables have a low level output with "pre-emphasis". This will not work directly into a sound card. You will need to hook up the turntable to a phono input on a receiver/integrated amp and then find a line level output on the receiver to send to the sound card. A tape output works good for this. Most likely, depending on your sound card, you will need a cable with 2 rca plugs on 1 end and an 1/8" stereo mini plug on the other end.
Of course you could also use a phono preamp and do it the same way, but most people have a receiver around and not a phono preamp.