There are a number of ways to set the origin on a set of files.
As a rule, setting origin at the lower left corner of a board will produce the most optimized Gerber files.
To set the origin to the lower left corner using the board outline:
Use the Auto (Center + End) snap found in the View menu, move the mouse to the lower left corner of the outline until it snaps to place.
Left click the mouse and you're done.
Setting a new origin to a specific coordinate away from the current origin:
The easiest way to do this is to manually enter the new coordinates on the command line at the bottom of the screen.
For example if your units are in inches and you want to move the origin point 600 mils below and to the left of the current
origin simply type in -.600,-.600 on the command line.
Another easy way to do this is by setting the grid (Setup / Set Grid) to .600" x .600" and the snap (View / Snap) to grid.
Then selecting Set Origin from the Setup menu, then move to -.600x & -.600y and left click the mouse button.