ITunes won't let me delete songs from my iPhone 3G?

I've been looking this question up for almost an hour now.
I want to delete all my music off my iPhone (because I'm giving it to someone else). But it won't let delete it. It won't even let me uncheck my music. I've synched it so many times. I think I know what to do, i've just forgotten how to get there.
How do you find the "manually manage songs" part? Because I probably don't have checked or whatever.

Simply factory restore the iPhone, that will wipe everything and then it can be passed to someone else.

Manually manage songs is found in the menu car>
Edit>
Preferences.

The alternative way is to plug the phone into your itunes account, click on the iphone once it is loaded, now select 'restore' or enable disc usage and then 'restore'

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