Okay, this is a little messy, but I'm at work. Feel free to ask questions, and other people here, feel free to disagree with me or join in.
What you need to do:
1.) Get a "Digital Audio Workstation."I'm sure other people in here will have their opinions on this. Personally, I love Logic Pro, but maybe you're not a Mac man. Pro Tools is, obviously, another (more expensive) pro choice. I'll jump ahead right now and say that Logic Pro comes with a decent set of everything I'm about to go in to detail in this post. Well, a nice "starter kit" anyway. It comes with some synths and samplers, the tools for editing MIDI, and a decent set of additional plugins for mixing and master (reverb, EQ, etc). But let's slow down and move on to the second thing you'll need.
2.) Know what MIDI is. I really hope you do, because it's always to awkward to explain.
http://en.wikipedia.org/wiki/MIDIMIDI is like music written in a language computers understand. Note on, at this loudness, for this length of time. If you're writing music on a computer, you're using MIDI as the base of it all. If you've ever used TabIt, Finale, Sibelius, or any other kind of notation software on your computer, then you've used MIDI. If you've ever played an electronic instrument, like an old casio keyboard, then you've also used MIDI.
3.) Build a "sound library." These will more or less come in two formats:- Synthesizers. I'm sure you know what these are. They're synths. Waveforms, altered and combined to sound neat. Great if you're doing electronic music, obviously, but if you're not...
- Samplers. This is the good shit. Samplers are how electronic composers write music on their computers that sound like real instruments. Here's how an EXPENSIVE sampler works: a company that makes samplers hires an orchestra, and they set up to record. They tell a violin player to play middle C as loud as possible, by himself, and they record it. They take that recorded audio, and map it to a MIDI keyboard, so when you press C on that keyboard (as hard as possible), that sound byte of the violinist plays. Then they ask the violinist to play it a little softer. And softer. And softer. Then they ask him to play it pizzicato, as loud as possible. Then softer, and softer. Then they ask him to play it all the other ways you can possibly play that note (with the wood of the bow, marcato, vibrato, everything ... depending on how much that sample library is going to cost...). And they record every one, individually, and map it to that MIDI keyboard. Then they go on to every other note. Then they go on to every other instrument. Then they take a lot of time and effort fine-tuning it so it sounds semi-natural out of the box. This basically means you can use MIDI to compose a symphony, and the sampler will play back the samples and ... it will more or less sound like a real symphony. Again, these can be very expensive, depending on the kind of quality, range of playing styles, realism, and features you're looking for.
A CHEAP sampler will use synthetic techniques and waveform editing to make it sound "close enough," but come on ... you can tell it's not quite there. Technically, General MIDI and old soundfonts are "cheap samplers." They're trying to emulate real instruments, but their sounds are often created synthetically, not by straight up recording real instruments.
Both synths and samplers will come in the form of what's called a "plug in." In your digital audio workstation, you'll have tracks. And on that track, you can put in some MIDI data. And per each track, you can "plug in" your audio source. You plug in the synth for that track, you get synth noises. You plug in a sampler, you get your sampled instruments.
Most people/companies that make sample libraries also make their own sampler plugins, which will be included. I use
East West right now. I've had it for a few years, and it is getting dated, but they have some great libraries. Feel free to browse the site and check out some examples. They still have some fantastically really sounding libraries.
Other companies, like
Kontakt, make a sampler and offer some libraries of their own, but other people also use their player. Thus, you actually might be able to find an old version of the Kontakt sampler for free. However ... the quality of sample libraries won't match what you'd get with a company like East West.
Oh, and if you get yourself a mic you can of course always just, you know, record yourself playing a real instrument.
4.) Learn how to fine-tune MIDI. Samplers won't sound perfect out of the box, especially if you manually enter in each note. If you enter the MIDI data in by pressing record and using a MIDI keyboard, then it will sound more natural ... if you're writing a piano part. You have to know how each instrument is supposed to sound and be played, which you may have an edge on already, and learn how to represent that in MIDI.
The biggest mistake people make with MIDI is making every note perfect, both timing-wise and velocity (loudness) - wise. Real music isn't that perfect, so you have to nuance it. Real music doesn't have perfect timing, and a real musician doesn't hit every single note with the exact same volume every time. Keep this in mind, but for god's sake BE SUBTLE.
5.) Get some mixing/mastering plugins, and learn how to mix/master.This is the most daunting part of it all. Since you're doing it all on the computer, now, there's no reason why you can't just do everything yourself, mixing and mastering included. Like I already said, Logic Pro comes with the tools to do this relatively well, but you should still take the time to learn how EQing works, and how to do it when mixing multiple instruments (always CUT, don't boost). Learn how and when to use a compressor. Learn how to use reverb. Etc etc.
Biggest tip: compare your music to other music. Side by side, right in the DAW.
Another tip: when mixing music, every instrument should try to have its own "space," as each instrument has a "sweet spot." A range, sonically, that's most important to that instument. Find that range, and cut out everything that's not essential, because even if there's just a little non-essential noise from that instrument, it could get in the way of another instrument. Cut, cut, cut. Don't boost.
6.) Practice