Help needed with coding

Im very new at vb2012 and i have compiled this code so far but need the outputs to be in a currency format any hints or comments are welcome and greatly appreciated thank you.

Public Class SalesTax

Private Sub Label1_Click(sender As Object, e As EventArgs) Handles lblMarkedPrice.Click

End Sub

Private Sub TextBox1_TextChanged(sender As Object, e As EventArgs) Handles richtxt1.TextChanged

End Sub

Private Sub Label4_Click(sender As Object, e As EventArgs) Handles lblNetPrice.Click

End Sub

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load

End Sub

Private Sub btnCalculate_Click(sender As Object, e As EventArgs) Handles btnCalculate.Click
Dim i As Double = richtxt1.Text
i *= richtext2.Text
richtxt3.Text = i.ToString

End Sub

Private Sub btncalculate2_Click(sender As Object, e As EventArgs) Handles btncalculate2.Click
Dim i As Double = richtxt3.Text
i += richtxt1.Text
richtxt4.Text = i.ToString

End Sub

Private Sub btnClear_Click(sender As Object, e As EventArgs) Handles btnClear.Click
richtxt1.Clear()
richtext2.Clear()
richtxt3.Clear()
richtxt4.Clear()

End Sub

Private Sub BtnExit_Click(sender As Object, e As EventArgs) Handles BtnExit.Click
Close()

End Sub
End Class