Private Sub RichTextBox1_TextChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles RichTextBox1.TextChanged

Dim index As Integer = Me.RichTextBox1.Find("tutorial") '
If index <> -1 Then
Me.RichTextBox1.Find(RichTextBox1.Text)
Me.RichTextBox1.SelectionBackColor = Color.Yellow
End If
End Sub