Okay, music is sorta my meat and drink, so I can't help but reply to this, even though I've been silent on here for a very long time.
When I was doing music stuff in college, I was taking some composition classes, and some independent studies and such. My instructor pretty much said, Composition doesn't follow theory, it breaks the rules. But theory is important because you need to know which rules you're breaking, and why.
For simple midi writing, I would recommend cakewalk, especially if you're using a midi compatible keyboard. It's relatively cheap, and I have homestudio 9 myself.
If you're more focused in writing scores and written music (i.e. intending to print it out for live performances) I would say try finale, although it can be a little unfriendly at times, and it's pricey -.- I'm using Finale 2k1, but I got it from a friend *cough*
If you're into mp3 type stuffs, well, I can't really help you there. I have some decent soundfonts, and I play around with Fruity Loops quite a bit, but I don't think I'd call my mp3 versions "professional" sounding.
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It's been awhile since i've written anything significant, and i'm not saying i have gobs of time to work on music, but if anyone here needs a composer, I wouldn't mind trying to get back into writing stuff. A little game music might get me back in the swing of things ^_^ Just realize I tend to be slow at writing stuff.

I'm supposed to help a friend of mine out with some relatively simple music sometime in March. Apparently he's going to need a composer for his student projects at Full Sail.
I'll try to put together a portfolio of sorts later.
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