How do I choose a specific page to be the first page that site visitors see, and not a module?

In other words, how do I set the default page?

I have seen this question come up alot in all the CMS systems I have used. People get too reliant on modules and information stored in a database. If you have static content such as an About Us page that you want to display, you don't need HTML wrappers or a special module. If it was a normal website you would just create a new page with the same menu items as your index.html page. You can do the same thing here. Just make a copy of the index.html file in your theme folder. If you are using a documented theme, you should see a comment line that says "Display center blocks" or something similar. Just replace this entire section with normal html static content. Name your new index.html file to aboutus.html or whatever applies to your page and add a link to this page in your main menu. This is much easier than other methods if you just need a page or two of extra content.

edit- I realized the menu portion probably is not a beginner thing to do. You are not stuck with the automatically generated module menu. You can create a custom block in the admin menu and design your own menu. You just add the same links that you want to your custom block plus any extra links you want such as our about us example. When your custom menu is working just turn off the display option for XOOPS main menu block in the blocks admin.

I probably should've been more specific.

There is a line: $xoopsConfig['startpage']

How do I define startpage variable?

I just read your question again. I think I misunderstood it and gave you a long answer. In admin under general preferences you have an option to choose which module loads at startup. I think it defaults to NEWS. Just pick NONE from that dropdown list. If you want to add some content to the new blank homepage, you can do so by creating a custom block that only displays on the homepage. Make sure you set group permissions so anonymous users see your new custom blocks. That's what I did for my start page.

Sorry, I gave a long answer that first time. I misunderstood.