Prevent multiple cells selection & Lock Cell

For the protected cells when protecting you a list of options one of which is to allow users to select locked cells, so you need to make sure all other cells are unlocked (Ctrl+A then right click, format, protection, uncheck the locked box, then lock the individual cells you want to protect) as locking cells only works when a sheet is protected.

The select only one cell can be cured like this:

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Target.Cells.Count > 1 Then
MsgBox "Please select only one cell", vbOKOnly, "Multiple cell detection"
Range("A1").Select
End If
End Sub

put this in the worksheet code module (right click worksheet tab, view code and paste in)