1. Letting Excel know which cell needs to be blocked. This is done by going to the final tab of "Cell properties" (Ctrl+1). By default all cells are blocked, so the fastest way would be to select all cells (Ctrl+a) and uncheck the blocked box. Then select the cell you want to block and check the blocked box.
2. Protecting your sheet. Go to the "Review" tab to find the option to protect your sheet.