Here are some questions:
-is this really open-source ?
-is it easy to use for new users ?
-has it got a WYSIWYG editor ?
-about the forum modules, are they free and opensource too ? Can you suggest me a good (and easy) one ?

I'm trying out many CMS but all seems so confused to me: building a website become an odyssey for me.

Hi,

welcome to xoops.

-Yes it is open source ... really!
-It depends, but you may ask. also some usefull readings:
http://xoopsdocs.net/modules/docs/index.html
https://xoops.org/modules/newbb/viewtopic.php?topic_id=57133&viewmode=flat&order=ASC&start=0
-No, no WYSIWYG, but it has most of them as addon packages or modules. You may need to change some modules to make use of them ...
-All modules that use XOOPS functions are should be open source! There are very few exceptions. CBB is one of the new forums. You need to install also frameworks and xoopseditors package. Other forum modules are d3forum, newbbex ...

frameworks and xoopeditors ?
I saw many modules, but I need a tip: an easy and free one (or a short list) don't be generic please.

Some features like hightline and click to quote/url because some forums just trow [quotes] and emoticons at the end of the text, and you have to copy and past them if you want to insert them in the middle of the text...do you understand me ?

EDIT: omg, the installation seems complicated...I hope running the site won't
BTW I can't download frameworks...

Hi,

I'm sorry, but I do not use forums in my sites - only on one site I have the most basic newbb1. So I'm not very shure about this and someone may correct me, but I think what you need is functionality of the editor, not the forum itself. So you will need one of the more advanced editors.

I think your best bet will be to use CBB (https://xoops.org.cn/modules/wfdownloads/singlefile.php?cid=8&lid=556) forum module (see it in action here: http://xoopsforge.com/modules/newbb/) with frameworks and xoopseditor package. Frameworks and xoopseditor are not modules but addon to core of XOOPS and they add some functionality, used by ppphps modules. You can download frameworks here: http://dev.xoops.org/modules/xfmod/project/?frameworks

and xoopseditors here: https://xoops.org.cn/modules/wfdownloads/visit.php?cid=1&lid=555