It sounds to me like you want to use an iframe.
I use the div to set any style needed. The name 'content' is needed for your menu to work. You can set the width and height to suit your needs. The source (src) would be what you want to show initially in the iframe.
Now for the menu: each menu item would need code similar to this
<a href="http://www.mysite.com/mission.html" target = "content">Mission</a>
<a href="http://www.mysite.com/calendar.html" target = "content">Calendar</a>
<a href="http://www.mysite.com/contact.html" target = "content">contact</a>
<a href="http://www.mysite.com/myspace.html" target = "content">MySpace</a>
Notice the target="content" part is always the same, since the target space is your iframe that was named content.
If the menu item MySpace refers to the website of the same name, I am not sure that you can do that, so you might need to modify the href statement to have the visitor actually leave your site.
Hope I understood what you are trying to accomplish.