Protecting a sheet in Excel involves two things... worksheet protection and cell locking. Cells are locked by default, so you need to unlock the ones you want users to be able to work with, before you turn on worksheet protection. You might choose to select all the cells, unlock them, and then lock the ones with formulas... this will allow users in all but locked cells.
Then go to the Review tab and click Protect sheet, making sure you don't allow them to change formatting. The password can be left blank.