Here's how to secure your workbook with a password in Excel:

Using the File Menu (For Excel 2010 and Later):

1. Open your Excel workbook.
2. Click on the File tab in the top left corner.
3. In the backstage view, click on Info.
4. Look for the Protect Workbook section.
5. Click on Encrypt with Password.
6. In the Password box, type your desired password.
7. Re-enter the password in the Reenter password box to confirm.
8. Click OK.

Using the Review Tab (For Older Excel Versions):

1. Open your Excel workbook.
2. Go to the Review tab.
3. Click on Protect Workbook.
4. In the Password to open box, type your desired password.
5. Re-enter the password in the Reenter password box to confirm.
6. Click OK.

Important Considerations:

* Password Strength: Choose a strong password that is difficult to guess. It should include a combination of uppercase and lowercase letters, numbers, and symbols.
* Password Loss: Microsoft cannot recover lost passwords. Make sure you remember your password or store it securely.
* Limitations: Password protection restricts opening the file. It doesn't encrypt the workbook content itself. For stronger security, consider saving the workbook as an XPS file or using additional encryption tools.