Change Code From Function To Sub

I am trying to do is look at the first two digits of the number in column c (Range c5:c5000) if the number starts with 01 then I need the formula in column g = f5*12 if the number is 03 then I want the formula to be g=f5*24 I have several of these "cases" or "if" to put in can someone get me going a little further

Here is what someone gave me to work with but I'm having troubles getting it to work...I get a name error in the cell and I cant use this they way it is because the code needs to go into the sheet mod with a bunch of other code.Also when I use the Option Explicit the rest of my code wont work.

Option Explicit
Function re(Cl As Range, Src As Range)
Dim i As Long
Select Case Left(Cl, 2)
Case "01": i = 12
Case "03": i = 24
End Select
re = Src * i
End Function

=re(C5,F5)