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We've all been newbies once
The welcome message on this site is a "custom HTML block" which you can make yourself by going to the administration menu -> system admin -> blocks
Below the list of module blocks is an input form, where you can place text, HTML code or php code - depending on the type you select in the drop-down menu in the bottom.
After you have created the block and in the process, selected which modules it is visible in (or just on the front page), you can go to system admin -> groups and edit the "block access rights" for each group to be accessible or invisible to individual usergroups.
Happy XOOPS'ing