You need to unlock the cells which are for data entry and then protect
the sheet.

Per Erik

Select the whole sheet, do format>cells>protection and deselect locked,
while still select press F5, click special and select formulas and press
enter, repeat the formatting part but this time check locked. Press Ctrl +
Home, do tools>protection and select protect sheet
Save the workbook, this will lock the formulas and leave all other cells
unlocked.
Of course if you only have a few cells where you want people to be able to
enter data you might as well just select those cells and de-select under
formatting then protect the sheet. You can also hide the formulas. This will
only protect for mistakes and the average user, anyone with Google can find
out a way to crack the password