I don't exactly find anvilstudio to be that easy to use. I prefer Noteworthy Composer more (Google for it, there's a free trial version). Of course, you need to know the stave-based musical notation to use it, unlike Anvil Studio.
But before you do any of that, jdl, do you have any musical training to create something that people won't want to turn their speakers off for? I mean, I'm having sort of nightmarish visions of amateur karaoke nights.

(Or maybe it's more like those initial audition cuts on
American Idol.

) Besides, there are probably thousands of free MIDIs you can find on the Internet--save yourself the trouble of composing your own, unless you have the talent.
Finally, unless it's the only thing GameMaker (or whatever you're using your MIDI for) supports, I don't really recommend MIDI for music, especially if you're planning to download other people's. Its only advantage is size (tiny). The technology is relatively outdated when it comes to game music, and so most modern computers only have a software-based MIDI synth that only does a mediocre job of playing MIDIs. If you're serious about the music, I'd push for one of the MOD formats, or go all the way to a full digital audio format like OGG. Of course, they do require more skills (and in some cases more expensive, professional software) to create.