Start from the beginning.. Four things.
Is this an Amiga 500 and not an Amiga 500 Plus. [ Red light or green light also please]
Do you have the official Amiga Workbench disk. If so which version. Is it green with say 1.2 or 1.3 on it.
When you turn the unit over can you see what is called a trapdoor with a tiny slot. On the base and NOT the side. Can you carefully prize the slot open and if there is anything in there tell us what it is. We are probably going to remove that if its there.
One tiny point... I assume you have a mouse. Don't pull the mouse or joystick in and out while the unit is active. If they are games you are playing, which one in particular gives the error message and describe exactly what you are seeing... ie blue box with message or grey box with message.