Step 1: Go to "File"
Step 2: Go to "Info" and click on "Protect Workbook"
Step 3: Select "Encrypt with Password"
Step 4: Enter and re-enter the password you want to protect the document with
Step 5: Your document should now be protect and require a password anytime you wish to open it
Note: This method also works on Word, Power Point, or any other Microsoft document creator