How Can I Install A "Main" in my Distribution Box, such that I could turn the main switch to allow all power to be off while I'm away?
You almost certainly already have a main disconnect. Code requires it be very close to where the wires come to your house. Look by the meter. If you don't have one, adding one is a job for an expert -- stop reading now. :-)

That said, you can add an additional main disconnect to the main panel, if you think that will be more convenient. 

Know your limits. Getting electrical work wrong can lead to electric shocks or structure fires. 

When you find your existing disconnect, turn it off, ensure that no one will flip it back on, open the panel, and have a look around. (Don't touch anything without verifying that the wires are not energized.)

Each panel is a little different, so until we know what yours is, we can't give you exact instructions.