Need help with Cash Register Application in VB.NET

I am supposed to build a Cash Register Application using Select Case statement. The Sales Tax needs to be calculated for the
amount purchased. I am supposed to add the sales tax to the subtotal and add the subtotal to the total. The Select Case statement
was used to enter the TaxRate and I think I am supposed to multiply the TaxRate by the TaxValue. I am having trouble with the button Enter key and the button Total key. When I enter a number such as $25.00, the $25.00 will show up in the subtotal, but when I enter $25.00 again it freezes up. Please help. I am new to VB and I really do not know what the heck I am doing.

' Private Sub txtCurrentPrice_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles txtCurrentPrice.TextChanged

'(I DO NOT KNOW WHAT THIS HANDLER IS SUPPOSED TO DO)

' End Sub

PrivateSub btnEnter_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnEnter.Click

'This handler is supposed to add the current amount to the subtotal and display new

'subtotal.

Dim sngCurrentPrice AsSingle

sngCurrentPrice = Val(txtCurrentPrice.Text)

Dim sngSubtotalValue AsSingle

sngSubtotalValue = Val(lblSubtotalValue.Text)

Dim sngTaxValue AsSingle

sngTaxValue = Val(lblTaxValue.Text)



sngSubtotalValue += sngCurrentPrice

txtCurrentPrice.Text = ""

lblSubtotalValue.Text = String.Format("{0:C}", sngSubtotalValue)

EndSub'btnEnter_Click

PrivateSub btnTotal_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnTotal.Click

'This handler is supposed to use the subtotal to compute the tax amount.

Dim sngCurrentPrice AsSingle

sngCurrentPrice = Val(txtCurrentPrice.Text)

Dim sngSubtotalValue AsSingle

sngSubtotalValue = Val(lblSubtotalValue.Text)

Dim sngTaxValue AsSingle

sngTaxValue = Val(lblTaxValue.Text)

Dim decTaxRate AsString

SelectCase sngCurrentPrice

'amounts under $100

CaseIs < 100

decTaxRate = 0.05 'Sales Tax

'amounts between $100 and $500

Case 100 To 500

decTaxRate = 0.075 'sales tax

'amounts above $500

CaseIs > 500

decTaxRate = 0.1 'sales tax

'if no other case is true

CaseElse

decTaxRate = 0.0 'sales tax

EndSelect

'lblTaxValue.Text = strTaxRate

Dim dblTotalValue AsDouble

dblTotalValue = Val(lblTotalValue.Text)

lblTaxValue.Text = decTaxRate

'sngTaxValue += sngSubtotalValue

'dblTotalValue = sngSubtotalValue + decTaxRate

'dblTotalValue = sngSubtotalValue + (sngCurrentPrice * decTaxRate) 'or

dblTotalValue = dblTotalValue + (sngCurrentPrice * decTaxRate)

lblSubtotalValue.Text = String.Format("{0:C}", sngSubtotalValue)

lblTaxValue.Text = String.Format("{0:C}", sngTaxValue)

EndSub'btnTotal_Click