Will the music on your computer be deleted if someone puts their iPod with it that has been downloaded from itunes and tries to download onto iPod?

No, the music on your computer will not be deleted if someone puts their iPod into your computer that has downloaded music from iTunes and tries to download it to the iPod. iTunes will not delete any files from your computer without your permission. If the person tries to download music from your computer to the iPod, iTunes will copy the files from your computer to the iPod, leaving the original files on your computer untouched.