Two things have to be in place to protect a cell... The cell must be locked and then sheet protection must be turned on.

Worksheet cells are locked by default, so you need to unlock the ones that don't have formulas. You can select the entire worksheet and unlock them and then select the cells that contain formulas and lock them back. (Or you can Find & Select Constants and unlock them, but this won't unlock empty cells.)

To Unlock or Lock, go to the Format Cells and on the Protection tab, uncheck (or check) Locked.

Now when you protect the sheet, only the cells that are "locked" are protected from being edited.

To protect the sheet, right-click on the worksheet tab and choose protect Sheet. check any boxes that you want users to be allowed to do. There is no need to set a password unless you are concerned about malice. Click OK.

Very clear explanation Kimberly, but one question before I get started.

You say that by default a spreadsheet's cells are all locked. Dumb question. If all cells are locked, then how can anyone (including me) open a brand new sheet and enter anything in any cell? Doesn't 'locked' mean keep out?