I have Multimenu installed, but how can I copy a link from the main menu into the new menu that I made with Multi menu? And delete the link in the mainmenu...

I would say: copy and paste from the address bar in your browser, and delete the part before 'modules/etc'. (So the http:yoursite.com/ can be skipped.

Deleting a link from the main menu can be done, but I'm a newbie as well... I didn't have to use this possibility yet, and I wonder what your motivation is.
You can change the name of a link in the main menu in the system admin --> modules --> change name in the textfield under the icon.

Greetings,
Ronaldus

do delete an option from the main menu just set the weight to "0" this will make it so that it's hidden. The module will still be installed, but it will be hidden from the main menu.