Code:
Public Class Frmbilling

    Dim itemamount, tax, subtotal, total As Double

    Private Sub btnCalculate_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCalculate.Click

        getitem()
        calctotal()
        calctax()
        calcsubtotal()
        displaytotal()

    End Sub

    Private Sub btnClear_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnClear.Click

        Rdbcappuccino.Checked = False
        RdbEspresso.Checked = False
        rdbLatte.Checked = False
        RdbIcedLatte.Checked = False
        RdbIcedCappuccino.Checked = False
        TxtQuantity.Clear()
        TxtItemAmount.Clear()
        chkTax.Checked = False
        TxtQuantity.Focus()

    End Sub

    Private Sub btnNewOrder_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnNewOrder.Click
        Rdbcappuccino.Checked = False
        RdbEspresso.Checked = False
        rdbLatte.Checked = False
        RdbIcedLatte.Checked = False
        RdbIcedCappuccino.Checked = False

        TxtQuantity.Clear()
        TxtItemAmount.Clear()

        chkTax.Checked = False
        TxtQuantity.Focus()
        Txtsubtotal.Clear()
        TxtItemAmount.Clear()
        txtTax.Clear()
        TxtTotal.Clear()

        'CalcGrandTotal()
        'CalcAddOneCustomer()



    End Sub

    Private Sub btnExit_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnExit.Click

        End

    End Sub

    Sub getitem()

        Dim quantity As Double
        TxtQuantity.Text = quantity
        If Rdbcappuccino.Checked Then
            TxtItemAmount.Text = 2 * quantity
        ElseIf RdbEspresso.Checked Then
            TxtItemAmount.Text = 2.25 * quantity
        ElseIf rdbLatte.Checked Then
            TxtItemAmount.Text = 1.75 * quantity
        ElseIf RdbIcedLatte.Checked Then
            TxtItemAmount.Text = 2.5 * quantity
        ElseIf RdbIcedCappuccino.Checked Then
            TxtItemAmount.Text = 2.5 * quantity

        End If
    End Sub

    Private Function calcsubtotal()

        Dim sub1 As Double
        subtotal = sub1 + itemamount
        Return (subtotal)

    End Function

    Private Function calctax()

        Dim subtotal As Double
        If chkTax.Checked Then
            tax = subtotal * 0.08
        End If
        Return (tax)

    End Function

    Private Function calctotal()

        total = subtotal + tax
        Return (total)

    End Function
    Sub displaytotal()

        Txtsubtotal.Text = subtotal
        txtTax.Text = tax
        TxtTotal.Text = total

    End Sub
End Class

i dont know exactly where to go from here, i am getting 0 in all my textboxes, i dont really understand functions and subprocedures and byval and by ref, so can someone help me to figure out what i am doing wrong.
this is a site with the same project on it heres the link if it helps you to understand what i need to accomplish
http://www.shsu.edu/~mis_gab/MIS%202...Chapter8HW.pdf

thanks-