Hello can someone tell me how to fix my imac g4,

Try running Disk Utility from the install CD/DVD and have it repair your internal hard drive. If that doesn't cure it, also have it repair permissions as well.

A stronger tool for these file system repairs is Disk Warrior, which costs about $100. I recently used Disk Warrior to fix a colleagues Mac at work which had the same symptoms as yours. Apple's Disk Utility could not repair it. It's best to first try Apple's Disk Utility, however.

Do you have the Hardware Test Disk? I'd run those tests. Have you recently replaced memory? Was the Mac working fine until recently? What might have changed to cause a change in its behavior (e.g. replaced or added memory, etc.)

When you installed the Mac OS, did you do an erase and install? If not, something leftover from before might be corrupting the new install.

Precisely what model/make of iMac do you have? if it's on 10.4, I'm guessing that it is at least 8 years old or so. Hardware sometimes begins to fail even earlier than that.

Based on the tags, it looks like it is an 800 Mhz iMac G4. This machine may need taking apart to replace the PRAM battery. Bring it in for service, if that hasn't been done in the last 4 years.