Newbie to Macs, its outdated and slow

I am a New User to Mac. I inherited an IMac G5. Its very slow on games and video. I looked at other questions concerning the G5 but since I am not familiar with Macs, I am not maneuvering well enough to correct any issues. I did however find the disk utility and I repaired it (to much of my amazement). But I do not know how to clear caches, I do not know how to get from one place to another (very diff from Windows). So if anyone would help this babe in the woods, I would appreciate it, Thanks!!