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All the answers you need are here on the forums and you may find some comprehensive information by following the links in my signature.
To install a module, download, unzip and upload to your xoops_root/modules folder, the same goes for themes.
You can activate a module by going into the module admin area when logged in as an administrator (your initial user/pass you created when installing XOOPS)
You are highly recommended to follow and read the links provided below. These are very basic functions and all your question are covered in many different areas.
Also be sure to look for JackJ's XOOPS guide for dummies, a very good way to learn the system!
Good Luck