What is most important is that you use the GMail interface to your account, and you can even change some things, like the picture that appears by default of GMail in the top-left corner of the page.
You can also create a default iGoogle page for all the users of your domain, and share Docs, Calendar Events, etc.
To add this app, you have to own a Google Account, then google the word "google apps", open the first link.
There, put your domain in the top-right corner, and finally add the aplication, after some questions, and showing that the domain is yours, you'll have Google Apps in working for your site.
If you have some problems adding the application please post here, so I can help you, but as I said before, is just a simple process.
Here I leave you some pictures of the App with my site:
(It's in spanish, but you can get main idea)
Google Apps own Start Page
Google Apps own email account
I hope this was helpful for you, Greetings

