Hi David,
I was in the same situation you are now. I was totaly lost because even with the good and long explanations of the guys here in the forum i didn't know how the oacs system worked and how the pages were built in a big piece html code that we see on the browser.
How did i find it out?
I found that out just going on each page and understanding what each page does. Take your time, you will have the pages on your mind very soon, just need to pratice and read them. Remember always to read both together adpXtcl
The site-master page is the first one you should look.
Another good tricks you can do:
Always when you are in a page, go to the browser and click on view source code, sometimes we can trace where you are from there
If are the site-wide-admin, in the bottom of the page, always appear a square called profile information. This square show you the pages those have been called to show up on the browser, it's not all the page but most of them.
Every time you don't know what a variable really does, just post on the adp page of the tcl page the variable is related to. On the adp file you write down @variable-name@ and it will show up the line in the browser.
In this case it would be headerstuff: just put a line @headerstuff@on the adp file and refresh you page to see what is in there.
Let me know if that helps. =)
@header