Welllll........that's kinda a loaded question. Which things would you be talking about? About all musical things can be bought from sites like
www.musiciansfriend.com www.zzounds.com/a--2676837 www.fullcompass.com just to name a few. When it comes down to which things to buy.....that's a harder question to answer. There are many ways to set these "things" up. You could for example, get a PCI sound card such as this one
http://www.musiciansfriend.com/srs7/g=home/search/detail/base_pid/701376/ and hook up each channel to a channel in a mixer such as this one
http://www.musiciansfriend.com/srs7/g=rec/search/detail/base_pid/630051/ This would allow you to record many separate tracks (like if you wanted to record drums, or a live band, etc.)
You could also get some kind of USB or Firewire soundcard with fewer inputs like this
http://www.musiciansfriend.com/srs7/g=rec/search/detail/base_pid/701368/ This has preamps, and phantom power much like the mixer would have in the other option. You can only record two tracks at a time though, which kinda stinks.
You could go real cheap and just get a cheap preamp like this
http://www.musiciansfriend.com/srs7/g=rec/search/detail/base_pid/182482/ and then adapters at radio shack to go into the soundcard you have.....
All in all, this advice is merely scratching the surface of things you can do to record into your computer. I wouldn't take this post and go looking for things, I would encourage you to read more info on this site and others and learn about how things like this work. This post is really just kinda a jump start to look into the many many many possibilities and the thousands thousands thousands of dollars that can be spent on tons tons tons of equipment. Hope it is a little helpful and answers your questions some.....
if you want info on what mics, preamps, soundcards, and other equipment to buy, please read the multiple posts in the archives under newbies, microphones, etc. It would be very time consuming for me to try to scratch the surface of this, especially since these questions have been answered on this site before. There is plenty of good info here.....read and then read some more!