You will need VBA code to do this.
Right-click on the sheet tab name at the bottom of the screen, select "View Code", and paste this code in the resulting VB Editor window:
VBA Code:
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.CountLarge > 1 Then Exit Sub
' If "S" is entered in column F, jump to column M
If Target.Column = 6 And Target.Value = "S" Then
Target.Offset(0, 7).Select
End If
End Sub
Then any time you enter "S" in a cell in column F, you cursor will jump to column M of the same row.