A demo won't tell if you are good or not and is.. there are so many ways of manipulation (bpm counters, pc autobpm programs) that they can not rate the skills on a demo. Start to spin in a local pub or something and make people KNOW YOUR NAME and spin a particular style so after a while you can start local gigs. If you 're good (no trainwrecking ) then you can climb higher and start gigging some bigger partys and bigger... and bigger... Still the best way to become a dj I think. A demo could help a bit but bear in mind that every setup will be (slightly) different so you need some EXPERIENCE and you won't learn all you need at home.