If you can't work out how to use a class, or a property, or a method, the first thing to do is try MSDN:
Google:
"MSDN Math.Abs"[
^]
The result you want is normally near or at the top:
MSDN:
Math.Abs Method[
^]
This shows the various overloads, so pick the appropriate one:
Math.Abs(Double)[
^]
This has a good description, and even an example!
Dim absoluteValue As Double = Math.Abs(1234.7123)