Protecting cells in Excel is a two step process. First, you have to set the locked property of the cell. Cells are locked by default. To lock or unlock a cell, goto Format>Cells and then to the Protection tab.
Locking cells doesn’t do anything until you protect the sheet. To do that, go to Tools>Protection>Protect Sheet.
Generally, you’ll only want to unlock those cells that you want users to be able to change. Set up your spreadsheet the way you want it, then unlock the appropriate cells. I’ve added the Lock Cells toolbar button to my toolbar to make it easier.