how do i install a second ipod on one laptop and have it sync with only my music

I have my son's ipod on my laptop and when he plugged it in it auto sync and made all the changes to music, games, etc. I recently purchased an ipod for myself and went and purchased some songs. However it downloaded the songs into my son's playlist. How do I set up my own account so when I plug in my ipod it does the same as his plus allow me to download my own music.