This will add a row just below the active cell's row whenever any change (other than to delete) is made in column C. (Note that this includes editing an existing value...) Is that what you're looking for, or is there more criteria to consider?
The code goes into the sheet code module.
(Right click the sheet tab, choose view code & paste this into the white area that is the sheet code module. Press AltQ to get back to your sheet.)
Private Sub Worksheet_Change(ByVal Target As Range)
If Intersect(Target, Columns(3)) Is Nothing Then Exit Sub
If Target.Count > 1 Then Exit Sub
If Target = "" Then Exit Sub