First thing you want to do is Go to File / Page Setup and make sure
you're working on a letter-size sheet with landscape orientation.
Then you want to View the Master Pages and adjust your guides to show
the fold lines. It's a good idea to have the border guides set at .375"
(no less than .33" if this will be on a press, unless you're going to
make the color "bleed" off the edge of the sheet). Remember to consider
that you'll want space on either side of the actual fold; it easiest if
you draw guides in on the Master sheet so you can see where your
margins should be.
Those are the most fundamental things you want to remember; uneven
margins are often why some documents don't look quite as nice as
others.
