If you wanted to do that, to change whether a cell is "locked" or not, you would select the cells that you want to be able to edit while the sheet is protected, then right click on one of the selected cells and click on Format Cells. In the Format Cells window, go to the Protection tab and uncheck the "Locked" box. This box is what determines which cells will be affected by worksheet protection and is turned on by default. After you have unlocked the cells you would like to be able to edit, you would go to the Review tab and click on Protect Sheet (in earlier versions, it's Tools | Protection | Protect Sheet). In this window, you want to make sure that the checkbox at the top is checked, and then you can check the boxes in the bottom to achieve the desired level of protection you are looking for. You also have the option of adding a password if you don't want someone to be able to unprotect the sheet and edit it in an undesired way.
Just thought I'd add some instruction for protecting a sheet, good addition!