Midi is very tricky stuff. Yes, the first thing a noob to digital audio says is midi sounds so dry and artificial, how do I make it sound more realistic?
First thing is the sound quality itself. It seems like every month there's a new thread asking "What's the best synth to use for midi? Here's the latest one:
http://www.pgmusic.com/forums/ubbthreads.php?ubb=showflat&Number=283968#Post283968Please read all the different opinions on this question.
The next thing about midi is what Notes just explained. It takes serious work backed up by years of experience to take a raw midi track and make it sound real.
The easiest way is if you're a good player already and you record the part yourself as midi, not audio, using a midi controller. Then you go into the piano roll and other editing tools to massage it.
Midi controllers can be a keyboard, guitar, wind and sometimes more esoteric things. If you're not a good player and don't have a controller then you're stuck doing it manually and that gets tedious beyond belief.
I'm not trying to burst anybody's bubble here but you really can't simply load in a midi file and then tweak it to create some killer solo that sounds like a real big name player did it. Is it possible? Sure, IF you're already a killer player but maybe it's on guitar and you need a trumpet solo, then using what you already know you could do it. But, even then it's not easy if you're not already a midi expert.
There are lots of really excellent players who will show up on forums I frequent asking rookie questions about midi and it's hard to know where to begin because it's so complicated to explain.
Bob