I loose it when I reboot

[FloydMc]

I have recently purchased a PowerMac G3. It was working fine. I purchased acopy of 10.2 - I installed it - very slowly. At first I could boot at either the old number or 10.2. When I shut down and opened upagain the only thing I get is a floppy with a question mark on it. The comuer will take 10.2 but how can I make it keep it so I don't have to einstall (al several hour process) each time I want to use it. I am a Windows person andtis is my first attempt at anything mac. help plese.