Help wity ipod syncing!!!

Hello,
i have an iPod classic.
to get more songs, i wanted to sync it onto another computer that had lots of songs already.
if i sync it onto this computer, will al of my songs be deleted??
i tried making an "on th go playlist".
will these songs be deleted or was this pointless.
also,

if i get these songs onto my ipod, and the songs erase,
can i sync it back onto my computer and just get the songs back again???


please help.
im normally excellent with the computer, but im confused!!!!!!!!