As CubKid said, you'll need an adapter. The adapter needs to go from RCA split cable to an 3.5mm stereo pin. You can pick them up at Radio Shack for less than a buck. There are various formats for the adapter, you can probably even get a single cord that has the RCA connectors one end and the 3.5mm pin on the other.
Anyhow, after that all you'll need is your burning software (Roxio, Nero, CDMate, etc) and a program that will record the audio. Since you're probably not going to keep the tapes around in digital form, I wouldn't worry about compressing them to MP3 (unless you're really tight on hard drive space). There's a program I use called All Sound Recorder. It's relatively straightforward to use, and it's free. You just make sure your recording input is the line-in on your sound card (which you've plugged the cord from the tape deck into previously) in the Windows multimedia section, and then click record on the program. It'll ask you where you want to save the file, and what format to save it in. Saving it to .wav should suffice. Then once you've recorded one side of the tape, hit stop on the program and repeat the process for side B. Now you've got the two sides in seperate files.
Toss the files at your burning software and you'll be all set.