You'll need some sort of USB or FireWire digital audio I/O device to plug a mic or instruments into to record.

You'll need a mic and cables.

You'll need software.

You'll need instruments that can be mic'ed up, or run direct...or you'll need the software to create the background music.

This could be MIDI software that you can play notes from a keyboard into the computer. It could be looping software to play back basically pre-recorded loops (ugh!) It could be MIDI composition software.

It could be a combination.

You'll need a monitoring system other than cheezoid little computer speakers to hear the music reasonably.

You'll need to learn how to use all of this, and have the talent to create the music.

You'll need patience to setup and learn any software, and money to buy all of it.

You'll need to understand that until you get pretty decent gear, get good at using it, and learn how to record and mix well, you'll probably not get much more than a demo-quality recording that you can take to where?

Maybe to a studio so you can make your songs sound really good.

BUT...don't get discouraged. We don't know what instruments you play. How will you get drums, bass, guitars and keys in there? If you have, say, a MIDI keyboard workstation type of thing, and most of the instruments are sequenced, you could just dump those into the computer, sing over it, and call it a demo.
For that, you'd probably only need an interface device, a decent mic (and maybe preamp) and some "lite" version of multi-tracking audio software. It may be handy to get software that will allow audio AND MIDI, so you could always control your keyboard with the computer, and add more MIDI tracks than your keyboard may allow, if you need them.

To go further, you'll probably have to start researching the next thing you need, and be prepared to enter the world of never-ending needs, wants and upgrades :shock:

This is a VERRRRYYYY deep subject, and there are a TON of considerations, and a world of choices to answer your question definitively when all we know is that you have lyrics and a computer.

But, this is as good a place as any to start (and better than most).

Tell us if you play, what you play, what kind of keyboard (if any), etc.

Probably start with getting an outside sound into the computer with an I/O device, and the software, mic and maybe preamp.

Without some way of actually creating and capturing music, though, you're pretty much gonna be stuck to a cappella.

Hope this helps get you started.

Kapt.Krunch

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This is everything you need to get started (sans cables); one function-per-button layout, almost no routing, etc. You might also consider upgrading to one that records digitally if you have the funds -- this way you can keep all materials in the digital domain for uploading stuff to your Myspace (which you almost certainly will do) or for playback in your car.