First of all, it's best that if you want to share things back and forth from yomac and a friend's pc, you should use a real pc disk. I've had mac disks go bad on me in the past when i formatted them as pc.
(If it says on the box when you bought it "reformatable to pc as 1.44mb then you are ok)
Place the pc disk in your mac. If it says it can't read the disk but asks if you want to initialize it, say no if you have stuff on there.
go to apple's ftp site, or even website and locate "PC Exchange"
Download and install it. from there, when you highlight a disk and go to file and select initialize, you can select wether it should be pc or mac.
just one last note: be carefull about trading files. images, text stuff like that is ok, but program's can't be traded, nor can some documents such as word or excell unless you have the mac version on your mac or at least something similar.