You'll need a license from the MCPS as Kempie has linked to there. It's relatively easy - on their site you can apply for a AP2 license (I think) where you list the songs you're recording and the composers, detail how many copies you're pressing, how much you're selling for, etc etc. THen they send you an invoice based on that (don't worry, it doesn't come to that much) and you pay it and then you can manufacture your CD. No good CD replicating company will press a CD of copyrighted material without an MCPS license.
Bear in mind you have to deal with graphic design, mastering, recording, etc. to make a CD.
But you say demos - is this just to get more gigs? Then MCPS have another license called a Limited Availability License which is around �20 for 100 copies lasting less than half an hour. Then it's tiered upwards.
If you're asking a record label (big or small!) to put out your album they will deal with the MCPS for you as well as the manufacturing. But you can do it yourself!
If you need more advice on this please ask - I've put out albums for an artist and if I can do it, so can you!
One last thing - are you concentrating on old songs to avoid the copyright? Or just because you like them? Copyright is nothing to be afraid of and the MCPS will help you through it. Worth applying for a license just so they can say that it's out of copyright, or some of your tracks are and some aren't. Better than you researching when and if all the composers died!