The first thing to do is browse the email info in the FAQ section here. I think there are some articles there that will help: http://www.lunarpages.com/support.php. Check out the tutorials, too, and the Control Panel Manual.

Also search these forums for "default address" (with the quotes). That's the name of the catchall, and there is a lot of info here on how to set it up.

For UI, for webmail, both Horde (my choice) or SquirrelMail are provided. Go to cpanel > WebMail. You can choose which to use each time you log in. For POP3 or IMAP access, you can use any client you like.  Outlook 2003 and Outlook Express are two possibilities, and there are quite a few others.  Instructions for setting each one up are in the tutorials and FAQs listed above, and also here in the forums. 

If you're very new to email, the most step-by-step way to figure it all out is probably to go to cpanel > Mail and look at the options there that you might want to set.  Then for each one, if you don't know what it is, use a 2nd browser window to go look at the cpanel manual, FAQs, and forum (search) for it. Then just go down the list one thing at a time.