From the Menu bar select Tools - Protection - Protect Sheet...
Make your selections from the dialog box and enter a password.
All cells should be locked by default.
To be sure, before protecting the sheet, select the whole sheet (click the square between the A and 1 column and row markers) right click over any cell, select Format Cells... and select the Protection Tab.
The Locked check box should have a check mark in it. If it is a solid color, click to make it a check mark - click OK and proceed to Protecting the sheet.