I have a "shell" that I have created within flash that will be my main webpage
(homepage). I want to make all other buttons (pages) open inside this shell.
Like a movie screen that changes when you click the buttons. What is the best
way to achieve this?

The basic cut technique is to create a blank MovieClip and place on a layer on
the root timeline. In properties on stage call it something like page_mc.

Then for button actions use page_mc.loadMovie("PageSwfName.swf");

Then create the page movies independently.

Be sure to plan the size. Generally the shell is larger and contains space for
common elements such as the page buttons.

Look at this basic
http://www.hosfordusa.com/ClickSystems/courses/flash/examples/LoadMoviePages/Ex0
1/LoadMoviePagesEx01.php

For more complex page loading such as transition effects you need to look at
MovieClipLoader and two or three blank MovieClips being swapped around for the
transition effects.