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That depends on what and how you want to change the main page content. To set up blocks you would go to System Admin - Blocks. To set permissions for your content you would go to System Admin - Groups.
To add content itself you use modules such as news for news items, a gallery module for pictures and so on.
Give a little more information and I can give a better answer.
If you are new to XOOPS read this
Xoops for Dummies publication. Lots of good information there.
Also check the faq to answer some of your questions. And see some of the tutorials on my site to help get you started.
HTH.