Try looking at these two topics about making a demo together.
Generally though, your voice demo should include all your best voices in a wide range of situations fitting the voices. What most people do is take scenes from video games, tv shows, anime, and reenact a line or two. Or you could just make up your own lines that you think would sound great. Also make sure for the demo to not be too long and to have good quality. Producers will be listening to many different demos and you'll want to make sure they hear your best work in a minute or so. Having good quality is kinda obvious, producers won't hire you if your demo is filled with static or hear you clipping a lot, very off putting. Other than that, check out those threads I linked and happy acting.