How to add a password to an Excel file

You may want to add a password to a file if you are storing sensitive data on a USB drive, emailing it to others, or you just want to add an extra layer of security to a file on a shared network drive.

• Select Save As
• Specify the file name and location as normal.  If the file is an existing file, leave the current information in place
• Click the Tools drop down list
• Choose General Options…

• Enter the password(s) you would like to use. (You may use one or both passwords as required)

1. Password to Open – This password will be required every time the file is opened.
2. Password to Modify – This password will be requested when you open the file. Without the password you still have the option to select the Read Only button to open the file without saving privileges. 

• Click OK and reconfirm password(s).

Please don't forget the password you have allocated to the file though - here is the official Microsoft stance

"Excel lets you password protect your workbooks, and your worksheets. But, it's easy to forget or misplace your passwords. Unfortunately, if that’s happened to you, we don’t have a way to help you recover a lost password.
Excel doesn't store passwords where you or Microsoft can look them up. That's also true for the other Office programs that let you protect files. That's why it's always a good idea to store your passwords someplace safe.
Some third-party companies offer programs for unlocking files. For legal reasons, we can't recommend those programs. You can try them, but at your own risk."

http://office.microsoft.com/en-001/excel-help/recover-a-password-to-open-a-workbook-or-worksheet-HA102809703.aspx