In the Select Case, change str1stCharacter to str1stCharacter.ToUpper.

Select Case str1stCharacter.ToUpper()

Try this:
<code>
Dim strInput As String = ""
Dim str1stCharacter As String = ""
Dim strOutput As String = ""
Dim intStringLength As Integer = 0

strInput...

Try these:

RichTextBox.SelectionFont = New Font("Tahoma", 12, FontStyle.Underline)
RichTextBox.SelectionFont = New Font("Tahoma", 12, FontStyle.Bold)
RichTextBox.SelectionFont = New...