Public Class Form1
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button1.Click
        Dim meinsuchtext As String = "a"
        Dim pos As Integer = Me.TextBox1.Text.IndexOf(meinsuchtext, 0)
        If pos >= 0 Then
            MessageBox.Show("Suchtext beginnt an Stelle " & pos.ToString)
        Else
            MessageBox.Show("Text nicht gefunden")
        End If
        Me.TextBox1.Text = ""
    End Sub
End Class