There are a few ways to lock an Excel sheet. One way is to password protect the sheet. To do this, click on the "Review" tab and then click on the "Protect Sheet" button. In the "Protect Sheet" dialog box, enter a password and then click on the "OK" button.

Another way to lock an Excel sheet is to protect the sheet with a password and then lock the cells in the sheet. To do this, click on the "Review" tab and then click on the "Protect Sheet" button. In the "Protect Sheet" dialog box, enter a password and then check the "Lock cells" check box.

A third way to lock an Excel sheet is to protect the sheet with a password and then protect the workbook with a password. To do this, click on the "Review" tab and then click on the "Protect Workbook" button. In the "Protect Workbook" dialog box, enter a password and then check the "Lock workbook structure" check box.