I've been struggling with this idea myself lately being new to the whole music creation thing and having no real talent with any instrument (my 'talent' with my keyboard is mostly limited to randomly hitting keys and miraculously having it not sound like complete crap ^^; lol) but here is the solution i've come up with....
Use google to find a 'basic' midi of the song you wanna tinker with. In the piano roll turn on the note ghosting and play around tossing in new notes or even just replace the existing instruments with sound fonts or whatever you happen to have to see how they'd sound plugged into the original.
Now what i meant by 'basic' is it has the original song, just the original song, and not some one else's remixed version. Good example would be Saria's Song from Ocarina of Time. You can find pretty heavy remixes of it pretty easily, good for inspiration to see how other people did it, but horrible for doing your own from scratch. Now if you find one which has it stripped down to the basics of the base line and melody and such but only what is actually in the original, you are set. Ghost the notes like i said before and start playing around using that as a sort of guide to help you get an idea of where the notes are supposed to go.
One problem i've had with toying with midi files in FL Studio is some times you get these mono-track midi files where you have the base line, melody, everything.... all tossed into one instrument and thus it gets tricky to seperate. Some songs you can just cut/paste (tip on that in a bit) the notes into a new track, others you should just try to find a better midi depending how complex it is and what you need.
So lets assume you got a fairly decent one of Saria's Song (i love that tune, so addictive ^^; ) but its only done in piano all mushed into 1 track. Not really a biggie since the melody and baseline are fairly well separated by a full octive or two. So you control+click/drag (or use the selection tool) after zooming to best fit so you can see them all at once. You cut.... you pop in a new track of a flute or whatever (assuming you are yanking the melody) and paste then get all happy and ah crap! The melody starts on bar 1! Thats no good. Sure you can always undo and copy/paste then line it up with the ghost of the original then delete the original notes but that sounds like too much work to me So just slap down a note on the first beat of the first bar and then cut/paste and it works perfectly!
Confused why it works? Its simple, when you didnt have that first note in, FL just stuck the pasted in notes all the way to the left of your selection (generally speaking you wont have one in this case so it'll goto beat 1 bar 1)... but when you stuck that little note in there, it told FL "Hey, i want this note in the start.... and then i want this huge delay then these other notes" so it does just that..... just like last time it shoves everything to beat 1 bar 1.... but the 'dummy' note acted like a place holder so now it lines up still! Just delete the one dummy note and now you slit it into 2 tracks. Much more convinient than trying to line stuff up all crazily ^^
Of course if you cant find a midi of the song in question and cant even find sheet music to help you out.... well you're still basically screwed at that point, try a mp3 to midi converter? Yet to have one of those programs give me even half decent results but in this case it might be better than having to blindly play trial and error guess work.
Then the hard part of figuring out how to pick the samples/sound fonts that you want and writing up the music starts buuuuut thats a different matter all together. One thing i'm finding which might be helpful though is when it comes to sound fonts, download everything that sounds like you might be even remotely interested in using some day (and even most of the things you dont care about immediately)..... even if they are low quality and overal crap, you will be amazed how well they can still be used if you toss in some reverb type effects or maybe use it as the carrier for vocoder and you can make that 'crap' into something rather interesting.
