How can I keep a cell from being changed by other people?

1. Select a cell/range;
2. Format - In format window select Protection tab;
3. Check Lok when you want to protect selected cell(s), uncheck when you want to allow changes. By default Lock is checked for all cells. You can also hide formulas here - formula is not displayed in edit mode;
4. Protect the worksheet.

It's a two step process. By default, all cells are locked. If there are any cells that you would like editable by others, you need to unlock them at Home:Cells:Format:Format Cells. Go to the Protection tab in this dialog and uncheck "Locked". If you do this a lot like I do, you will want to put the Lock Cell toggle button on your Quick Access Toolbar.

Next, you need to protect your sheet at Review:Changes:Protect Sheet. There are several protection options you can enable/disable. You can add a password if you want. If you don't, anyone can turn protection off. I usually don't use a password unless protection is critical. If you forget the password, sayonara worksheet. Again, if you do this a lot, you can add the toggle to the Quick Access Toolbar.