Margaret,
It's on the "Save As" dialog box that pops up when you
do a "File/SaveAs".
It's in the upper right hand corner of that "SaveAs" dialog box
and when you click on it (Tools), there should be a choice
for "General Options". Within the "General Options"
you can choose a "Password to Open" and/or a
"Password to Modify".
At least that's the way it is on Excel 2000.
If the above doesn't work, please post back with what
version you're using.
John