How do i make Mail my default email client?

Hi,

Currently running OS X 10.2.8 and have been using Microsoft Entourage for the previous few months. Only just started to use Mail that i got with OS X and it is much better.

There is one thing i need help on, in my address book when i choose somebody to send an e-mail to it opens up Entourage as opposed to Mail as it appears that Entourage is my default.

How can i make Mail my default e-mail client?

Help much appreciated.