Getting my Mac Mini ready to sell.

Good Afternoon

We are trying to get our Mac mini ready to sell OS X 10.6.8 but we want to clear all the data off of it before we sell it.

We do not have the original OS disc that it came with.

The machine has not been plugged in for at least a year and now it won't even start up, it comes on, makes a noise as if it is starting up, we get the apple on the white screen and the buffering symbol but the progress bar at the bottom gets to about 50% and then the machine turns off.

Think the machine was made in 2002 and we have had it for about 7 years.

Any help would be great

Many thanks

Chris & Diane