problem completely wiping my hard drive prior to selling my iMac

hi guys possibly me missing something obvious but when i go into disc utility to wipe my iMac completely. erase is greyed out.never experienced this before.
i am administrator and only user but name of mac is my daughter, does this make a difference? and does she need to de authorise it on app store
this was my first mac, then it was used by my daughter recently returned
any suggestions gratefully received.
iMac 2008

Take a look at this site.
http://mashable.com/2015/10/01/clean-install-os-x-el-capitan/#iQuGniiWpEqn

Do you have the discs that came with the computer? If not, you may want to purchased Snow Leopard (Mac OS 10.6) retail disc.

https://support.apple.com/en-us/HT201065

thanks for link, but problem is erase in dis utility is greyed out so wiping drive is not possible

You cant erase the drive while you're booted up and logged in to it. It cant be "in use".
You need to boot into an installer drive or USB stick before you can erase it.

As said a boot drive cannot erase itself. The usual process is to use an Operating System installer. Here is the process as described by Apple which also includes steps to disconnect from various Apple services which will save you and the new owner a number of problems, View attachment Selling used Mac.docx.zip

Alternatively, you can download GParted tool from http://gparted.org/download.php, create a bootable CD from the ISO, boot from that and then erase the Hard disk.