Always do a 'man <command>' to see the instructions and how to use them. In case you are still muddled up.
copy /home/j/mp3's to /home/mp3's
then you would 'cp -R /home/j/mp3's /home'
The '-R' flag stands for recursive which you will need if you are planning to move or copy directories. So, in this case you are copying the 'mp3's' directory to /home. Hope that helps a bit...