Hmm sibelius.. it seems pretty popular around here. I don't know much about this soft but at least I know you can use VST (virtual instruments) to make it sound better : http://www.johnglossenger.com/TIPS_FOR_ ... LIUS_6.htm
(maybe not with the student version ?)
To make it simple, you can have two (more in fact but wathever) kind of music, the one recorded (wav, ogg, mp3, etc.), and the one which define the notes themself, the one which the computer play "live" reading the file, midi (.mid).
Midi sounds often very cheap (the sounds doesnt "exist" but the soundcard define it, even it's generally the same everywhere).
Hopefully, you can "plug" virtual instruments of midi, and the notes will be played through them.
Free good soft that do the full thing (VST host, pianoroll to compose, effects, etc etc.) are for instance Macaw : http://code.google.com/p/macaw/
and LMMS : http://lmms.sourceforge.net/
(work as well with Windows) this last one has a good wiki and some interesting ressources into it, a community etc. : http://lmms.sourceforge.net/wiki/index.php/Main_Page
VST are around the web, plenty of website with them, some are free, some not.
If you prefer the "sheet music" composing and can't than the pianoroll one, it's also possible to create a midi with Sibelius, and then import it with another software to make it "sounds" good. The two I linked you can (I think) also handle your recorded stuff, you can put it into them, playing that along your composition.
And indeed, too many people get confused at start because they try to follow what they hear, made by experienced or famous composers. Keep it simple. When you'll overcome your little territory, then try to get a bigger one, more complex, little by little. But take your time, no one is after you and simple songs when neat enough are more popular than real complex works etc.
Good example, the song which got the most plays on my Soundcloud and by really far, was a 30 sec music box track (here : http://soundcloud.com/ziassan/sherlock-music-box
Maybe it could help you, here is a .zip with really simples (and old) works : they are the ones I made years ago, just staing with a terrible midi soft, not knowing well music and stuffs, but keeping it simple (and well, most of them are... wathever) : http://dl.dropbox.com/u/2379296/midi.zip
(I added some random midi I found next to those old compo).