First, if your both gonna share, you need adjustable bindings, but go try them on, they are not all the same. Find what feels the best.
As far as mounting, the board has threaded inserts in it. The bindings just screw down into the inserts. The bindings will come with the mounting hardware.
An easy trick for finding which foot to put forward is to stand shoulder width apart and have someone push you backwards. Whichever foot you move back first is dominate and should be in front.
As far as stance widths, you and your girl will probably differ, but usually shoulder width apart. Jump up in the air and look at your stance when you land. Transfer this width to mount your bindings.
Also, starting out, you will need to play with the outward angle of your bindings (duck stance footing). Its usually easier for beginners to ride with their front foot angled slightly, and the back foot at 90 degrees. Eventually you will work to a symetrical stance, which will make it easier to ride switch (opposite).
Just make yourself comfortable, if your not, your riding will suffer.
The biggest help you can give yourself is to read the trick tips and learn how to progressively edge. EVERYTHING builds from this, if you can't do this correct, you won't be able to do much of anything.