Erase Mac of all stuff

I got a black Macbook that has version 10.5.8.

I haven't used it in a few years and figured I'd give it away to my nephew that is now old enough to play around with it. One problem is that the battery doesn't charge but not much I can do about that...

What I do want to do is to clean out all of my stuff. That needs to be done even if it just goes to the dump.

I've tried to go into Disk Utility and Erase but all the options are greyed out so I can't do anything there.. How do I get around that so I can get in and erase the computer?

The options I have on the left are

465.8 GB WD5000

HD

HL-DT-ST DVDRW GSA (This has a CD disk icon next to it but no disk in the slot).

Neither of these three options will let me do anything (but erase free space) on the Erase tab.

Any suggestions?

Thanks so much

Sophie