I've started XOOPS just 2 days from now. I have a small background in PHP,CSS,etc. codings.

I can't figure how I can make a new page, maintaining the frames, themes, etc. Now, the only page I have is the index.php(the home page itself) and I don't even know how to edit it.

xoops is a dynamic cms and the new version 2.0.16 release with NO MODULE inside so you should upload some modules.
its totally belongs to your needs. if you want to have news or articles you can use "news" module or "article" or "smartsection".( search this site for a download link)
if you want to have some static pages use a module called "pages" ( again use search).
also i highly recommend read this new help document about xoops:

Starting a new XOOPS site - by skenow