If you don’t already have a turntable, consider one of the USB turntables.
www.ion-audio.com/ittusb.php
It will simplify things considerably.

Sean’s note: To convert your old LPs into digital format, there are a couple of things you’ll need: conversion hardware, and software to slice up your capture files into individual tracks (and clean up the sound). There’s one handy little bundle I like to recommend, the ADS Tech Instant Music (http://www.adstech.com/products/RDX-150/intro/RDX-150_intro1.asp?pid=RDX...). It comes with an analogue-to-digital hardware converter that connects to your computer via USB, as well as a software bundle that helps you clean up the audio files and convert an entire side of an LP into individual audio tracks.

The most important thing you’ll need to consider is how you’re going to get the turntable connected to the conversion hardware. Because turntables typically have a very low line level, you can connect it directly to the inputs on the converter module, especially if the turntable you have requires a grounding wire…you’ll just end up with a quiet, overly noisy signal. If you currently have your turntable plugged into a stereo, see if there’s a way to pass the audio back through the stereo and into the RCA inputs of the converter…that way you can often regulate the volume level of your capture by tweaking the volume knob on the stereo. Alternately, if you’re planning to do a lot of converting of vinyl-to-mp3, and your stereo and computer are a ways apart, you might want to consider buying a special pre-amp module that will sit between the turntable and the converter module — this Pyle pre-amp is available for about $25 at Amazon: http://www.pyleaudio.com/itemdetail.asp?model=PP999 — that way you can just bring the turntable over to the computer, and get the levels you need by connecting it to the pre-amp.