I can't play the Minecraft demo?

I tried opening up the Minecraft demo, and I'm signed in and everything, but when I click "Play Demo", it closes out and opens up a small window saying "Error: could not create the Java Virtual Machine. Error: A fatal exception has occurred. Program will exit." How can I fix this? I have played the demo on this account twice already, but my dad would like to see the game and how much I enjoy it so he can make his decision on whether or not he will buy me the full version.

You probably tried to allocate too much RAM to the game. This happens every time i try to allocate too much ram.