Hey Atlantis, Well the one thing I can tell you is be prepared. Yes a demo would be great but agian be prepared, be ready to belt out some vocals to this person on point, you need to be able to have confedince, its key make these people believe in your ability. If you go in there like hey I'm willing to learn have a positive attitude and say look, I'm just wondering if you could take a listen to this demo I had put together, basically just have a good attitude and also be able to take critisism. You need to take the bad feedback and use it to get better, once you find the bad in your work and focus on the good your going up. Another thing dont worry about the money it will just ruin you, if you start out worring about the music, and making good music, once you do that then the money will come. Now dont go putting together a quick demo and settle with it, only put out something that you know shows who you really are, because if you make something and someone asks you "on a scale of 1 to 10 how do you feel about this demo?" and you say anything but a 10 your a waste of there time and yours. you can go to anyone for advice take classes vocally anything but the fact is no one wants to fix someone, no one wants to make a bum a artist. you understand what im saying? everyone has a voice you just to find it, agian be prepared. practice makes perfect and less problems. :) good luck friend, I hope I helped you.>Rennagade^Rebel

You could also ask him for stems of a project, makes it easy to add your vocals in on what he is doing. You don't need to mix it much. Gives you a way to prime yourself up and get into the sound.
Kind of like Karaoke. You could practice and then take the stems to a pro studio and get them to track and mix your voice in on it.