substring and return the value after a specific character

you can use the split method to return the value after the colon :

   Dim word as String  = "Testing.BSMain, Text: Start Page"
   Dim wordArr as String()  = word.Split(":")
   Dim result as String = wordArr(1);

Assumes no error checking:

Dim phrase As String = "Testing.BSMain, Text: Start Page".Split(":")(1)

which simply splits the phrase by the colon and returns the second part.

To use SubString, try this:

Dim test As String = "Testing.BSMain, Text: Start Page"
Dim phrase As String = test.Substring(test.IndexOf(":"c) + 1)