How do I Reset an iMac early 2009 using El Capitan OS back to Factory Settings

Hello,

I've been trying for a couple of days to reset my iMac early 2009 desktop back to factory settings. Everytime I try doing one of the tutorials from Youtube I run across an issue the tutorial doesn't cover. So start over and end up with the same issue just using a different tutorial. All I want to do is wipe, slick, erase the HDD so I can give this thing away to someone who is need of a computer. I still need the OS intact and operational. In Windows we call this resetting wiping the machine. Here are the specs for the machine:

iMac early 2009

OS : El Capitan

Processor 2066Ghz Intel Core 2 Duo

Memory: 4GB 1060 mhz DDR3

Startup Disk : MacIntosh

Graphics Nvidia GeForce 9400 256mb