Networking Share Ubuntu to Ubuntu (local network)

[Richard]

Installed Ubuntu Linux on both Mac and PC,
now, newbie question: how does one share a volume hard drive,
Mac Hard 0 and PC Hard 0 (ie)

both are being share, and each machines, can see the other hard drive.
this is strictly between these two computers, I don't want no-one else 
from the outside,
having access, on these drives.

So, what step do I have to do.

Thanks +
Rich