Kathy,
I use ASA 400 film and in most cases always have it on the "Sunny" icon i.e. f/11.

To answer your second question use, use lot's of tape! Black Duct Tape.
I usually tape the top and bottom seams of the back and then I tape the metal bracket that holds the back on. Put it on both brackets.
I would also put a small strip of duct tape on the front, at the seams between the viewfinder and the rest of the body, on both sides. Make sense?
But then again, try it without it for a while too.

Double exposures happen! Heck triple and quadruple ones even happen too sometimes. I have one photograph I did several years ago, that was successful, and to this day I have no idea how many exposures are on it.

Where did you get it from anyway? What version do you have? S, CFN, N, What?
Check this out, it's a Holga Manual from Freestyle Photo:
http://www.freestylephoto.biz/sc_pro..._id=&pid=2681#

Welcome to the Holga club Kathy! Feel free to email me if you have any other questions!
And remember rule number of Holga photography.
HAVE FUN!
Brian