Computer sound cards (the ones that normally come with computers and cost anywhere from $20 to $95 separately) generally have a number of things in common:

• a joystick port. Big deal, you say? Well, guess what, 98% of the time there's a MIDI interface hidden in this connector. You need a special $20-or-so joystick-to-MIDI conversion cable to access it, but then you can hook up keyboards and all kinds of stuff right to your sound card.
  
• a mic input jack (with a 1/8" stereo minijack). Here's what you should do with this: nothing. It's not meant for music mics at all. It's designed specifically for computer mics. The fact that it has 3 connections should give you a hint that there's something weird about this jack. Pretend it's not there and don't plug a thing into it, especially a microphone! One guy who wrote to me was tearing his hair out in frustration because his recorded sound was so distorted; it turned out he was recording through his computer mic that he forgot was plugged in there...
  
  Some folks have written to me and say they're using computer mics for micing amps and such. Fine, if you're getting good results, but in general don't expect to use these for vocals or serious recording of any kind. Ditto even if you manage to luck out and find a soundcard that will actually provide a useful signal after plugging a regular professional mic into its mic input...the "preamp" in these cheap soundcards is crap anyway. If you want to record digitally, do it right!
  
• line output (usually with a 1/8" stereo minijack). You can feed this into a cassette deck or an amplifier (ideally connected to a good pair of near-field monitor speakers) for playback and mixing purposes. If you have a really cheap soundcard, it won't even have this and you'll have to get your output from the...
  
• headphone jack (with a 1/8" stereo minijack). You can use this for monitoring while recording, but don't use headphones for mixing.
  
• line input (usually with a 1/8" stereo minijack). This is the only thing you should plug any of your musical recording stuff into. 

  Unfortunately many people don't understand what "line input" means. It means an input jack at a line level. "Line level" is a standard value that is generally 200 millivolts and is compatible with normal stereo equipment such as CD players, cassette decks, stereo receivers, and so on that generally use phono plugs. It is not considered directly compatible with microphones of any kind, guitars, guitar amp speaker outputs, headphone jacks, or anything else that drives a speaker or headphone.

  Now, you can use your line input jack directly with a great many things, such as keyboard line outputs, guitar/bass amp line outputs, CD player line outputs...see the pattern here? You should only plug a line output into a line input, period. So whatever equipment you want to connect, just find the line output jack. There is, however, a sneaky way to plug a headphone output into a line input without blowing anything. Get one of those little inline volume controls meant for headphones, and you can use that to hold the otherwise way-too-loud signal from the headphone jack down to a level that will be reasonable for the line input.

  OK, but how then do you use a mic to record into the computer, or even multiple mics at once? You're going to need a mic preamp, because the signal from a microphone is way too weak to do much for a line-level input. Now, here's the deal: you can simply go out and buy a mic preamp, but the second you get another mic, you're going to need another one, so don't bother.

  Instead, buy a mixer. Not a cheap-ass DJ mixer that they sell at Radio Shack for $59, but a real mixer (like a Mackie, Behringer, etc.) with as many channels as you can afford. This kind of mixer not only has built-in preamps on all the mic channels, but should also have phantom power for condenser mics as well, which you will undoubtedly need at some point. And it will have a number of stereo inputs for mixing in keyboards or things such as effects processors, and possibly channel inserts, so you can plug in a compressor or two.

  Click here to find mixers on sale at Zzounds.com

  If you're on a low budget, you're better off with a proper, though more limited in flexibility, two-channel (minimum) mic mixer/preamp from a music store than almost anything from Radio Shack. These can cost less than $100.

OK, so how do you plug all those things into your sound card? Cables and adapters, dude. And if you think that's sarcastic, you haven't seen half the questions I have!