Need help wit data entry validation: IsValidISBN()

I've been working on this code but I cant understand it. I'm not a programmer, I'm a high school student who needs to pass his project.
This is my current code:

Private Function IsValidISBN(ByVal ISBN As String) As Boolean
Dim i, sum, cd As Integer
Dim checkDigit As Char
For i = 0 To ISBN.Length - 2
sum = sum + Val(ISBN.Chars(i)) * (10 - i)
Next
cd = (11 - sum Mod 11) Mod 11
If cd = 10 Then checkDigit = "X" Else checkDigit = cd.ToString()
If checkDigit = ISBN.Chars(i) Then Return True Else Return False
End Function

I need to modify it so that it will check the validity of an ISBN-13 number; particularly these 2: 9780306406157 and 978316148410
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