I take it you mean a demo CD of your band, so that you can send it agencies and record companies etc.?
You put three or four of your best pieces on it. How long they are isn't a problem unless they're very long, in which case, put them at the end. If the listener then gets bored, at least they'll have heard everything else.
Choose the pieces that best represent the range of your stuff, and plan it as you would a short gig. Think about how you would open it, when you would get people dancing or singing along or whatever.
If you can afford it, buy some studio time and get a properly mixed master, produced by someone who's not in the band and not biased towards any one of you (ie. not the drummer's dad or whatever). Yes, I know it's damned expensive - I've been there with my husband, but if you can stretch to it, then it's well worth it.
And good luck!