Don't listen to what what these guys have said, eastale. You'll be fine if you get a cord like this:
http://www.radioshack.com/product/index.jsp?productId=2110460&cp=2032058.2032228.2032247&allCount=36&fbn=Cable+type%2FAudio+cable&f=PAD%2FCable+Type%2FAudio+cable&fbc=1&parentPage=family . You'll plug the RCA end into the "tape out" on your mixer and that 1/8" end into your line in on your sound card (yes, even on a sound blaster.) Then, on your software, make sure you set your input to the line-in. Ok, then mic up your stuff, check your levels and press record. Everything will be on one stereo track, so it'll have to be a "live" recording, or you can record drums and then play the drums back so the guitarist can hear and and record him on a whole other track (if you have a multi-channel mixer software [you can find some free ones I believe]) for easier mixing. Then you can keep adding tracks as you want, but remember anything in the mixer being recorded will be on one track in your recording software. That's pretty much it, I did it for about a year when I first started, it will do what you want it to.
Now to all you guys that commented on this post: You guys have to realize that he is 14 years old, which means probably no job, so little money, this mixer will probably be a b-day present or something. You also have to realize that he's probably not looking to make killer productions, just get his instrument recorded. There's no reason that he needs better gear for what he wants to do. He'll be fine with this. Now, if he does want to create the next hit, then he will need better stuff, but until then, let him learn on this stuff. Good luck, eastale.