You need to make sure your worksheet isn't protected.
Then, right click on the cell(s) you want to lock and
select Format Cells/ Then click on the Protection Tab and
put a checkbox beside Locked.
Then, from the Tools menu select Protection and Protect
Sheet, OK. For some reason, whenever I try protecting the
whole workbook, it doesn't lock the cells.
Ctrl-a to select all the cells
Ctrl-1 and choose the protection Tab and unselect locked
Ok
Select the cells you want to protect
Ctrl-1 and choose the protection Tab and and select locked
Ok
Now protect your sheet (Tools-protection-protect sheet)