By default all cells are protected, but protection is turned off. If you turn protection on, then you would not be able to enter anything in any cell. So to begin with you need to remove protection from your entire sheet. Select the entire sheet by clicking in the corner or however you prefer, then select Format / Cells / Protection and clear the Locked checkbox.
Then you can enter your data in C5, format the cell to be locked by checking the same box and turn on protection for the worksheet under Tools / Protection.
If you want all that to happen transparently without your intervention, you would need to create an event handling macro.