Hii, I want to use FluxBox, but can't get it all right
for example How do I add things to my desktop?
how do I change the screen saver
and how can I add programs to my menu when pressing the mouse button?

I don't know how to change the screensaver, but only the menu is managed by fluxbox. You cannot add things to the desktop with fluxbox, but have to rely on third party tools like idesk or rox which handle icons.
To add programs to the menu you have to edit ~/.fluxbox/menu with the text editor of your choice. If you have questions about this look at the fluxbox website first, their documentation is quite good.

i used fbdesk for my desktop icons, worked ok most of the time.

To run programs at startup (which you will want to do for your desktop icon program and probably others) add a line in ~/.fluxbox/startup something like:

MAKE SURE YOU USE THE & AT THE END

Otherwise fluxbox will stop booting and wait for the fbdesk process to finish before it continues.

look in the startup file, the comments there are pretty useful.

As mentioned above fluxbox only manages the windows and menus. By looking in ~/.fluxbox/menu you'll be able to pickup the format for it quite easily and add your own stuff.

that'll give you a screensaver

As mentioned, you can do this by hand by editing the menu file, or you can automate it by

$ fluxbox-generate-menu

$ fluxbox-generate menu -B

will keep your backgrounds settings.