computer wont load from reboot?

Hi there i have a g4 mini i have just brought this from ebay the osx software on it was 10.4.11, i have downloaded the 10.5 leopard and tried to load it onto the mac but nothing happened i tried partianing it but that wouldnt allow me either. It froze whilst trying to put it onto the computer so had to reboot it, now all that happens is it brings you too the apple logo loading screen and wont go any further ?? Any ideas on how to fix it or even get it back up and running ??