Have you looked at the excel Help section which gives the folowing:
Set a password in an Excel spreadsheet
To encrypt your workbook and set a password to open it:
Click the Microsoft Office Button , point to Prepare, and then click Encrypt Document.
In the Password box, type a password, and then click OK.
You can type up to 255 characters. By default, this feature uses AES 128-bit advanced encryption. Encryption is a standard method used to help make your file more secure.
In the Reenter password box, type the password again, and then click OK.
To save the password, save the file.