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u could start with reading the FAQ and operations documentation.. which has useful information regarding using xoops.. or search the forums using the search feature looking for setting start page or homepage..
goto XOOPS admin, then system module, then preferences, then general settings and then find the option to select the module for start page..
it's a great idea to actually play with XOOPS yourself and go through all the admin section just to actually see what options are available, then you would probably have seen that option ;)
also if u make custom blocks and u use php code, make sure u select php in the block type when you make the block.. and remove the tags () as they aren't needed..