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xoops allows you to make most of the changes to your site through it's admin or 'back-end' area.
First thing you need to obtain is the username and password for the XOOPS administrator (sorry if you already have this, your post is a little unclear).
Once you are logged in as admin, you can add content, set preferences and make changes to templates etc.
Never let a man who does not believe something can be done, talk to a man that is doing it.