First, choose which cells are okay for people to make revisions in or to enter data in. To do this, select each cell or cell range in which you want to enable data entry.
Then, on the Format menu, click Cells, click the Protection tab, and then clear the Locked check box.
Just a word about Step one: All worksheet cells are locked (or protected) except for the cells you first unlock by clearing the Locked check box. You have to unlock the cells that you want people to work in (Step one) before you turn protection on (Step two).
Feel free to ignore the text on the Protection tab, which is instructing you how to turn protection on — this is exactly what you will do in the following Step two, so rest easy for now.
If you have a formula that you don't want others to see, select the Hidden check box (you'll need to have previously selected the formula cell in the worksheet). This option hides the formula from view in the formula bar so that people will only see the formula result in the worksheet cell.