Newbie help with RadioButtonList

Hi, I have only been at this for a few days, so please bear with me as I have no previous programming experience at all.

I am trying to create a simple Exchange Rate converter for our company.

Please find attached code

 Function getrate() As System.Data.IDataReader
 Dim connectionString As String = "server='(local)\netsdk'; trusted_connection=true; database='Exchange_Rate_Convert"& _
"er'"
 Dim dbConnection As System.Data.IDbConnection = New System.Data.SqlClient.SqlConnection(connectionString)
 Dim queryString As String = "SELECT [Rates].* FROM [Rates]"
 Dim dbCommand As System.Data.IDbCommand = New System.Data.SqlClient.SqlCommand
 dbCommand.CommandText = queryString
 dbCommand.Connection = dbConnection
 dbConnection.Open
 Dim dataReader As System.Data.IDataReader = dbCommand.ExecuteReader(System.Data.CommandBehavior.CloseConnection)
 Return dataReader
 End Function
Sub Page_Load(sender As Object, e As EventArgs)
if not page.ispostback then
rates.datasource = getrate()
rates.databind()
rates1.datasource = getrate()
rates1.databind()
end if
End Sub
Sub rates_SelectedIndexChanged(sender As Object, e As EventArgs)
 currencytxt.text = " " & rates.selecteditem.text
End Sub
Sub rates1_SelectedIndexChanged(sender As Object, e As EventArgs)
 finaltxt.text = ((value_to_convert.text / rates.selecteditem.value) * rates1.selecteditem.value)
End Sub

I have 3 questions:

Question 1:

Everything works as I expected it to if the user uses it once, Enter value -> click on first currency -> click on second currency.

However if they (Perhaps) realized they made a mistake with the first currency click again, then the "Finaltxt.txt value" is not correct.

What I would like to do is to allow as many different button clicks from both sets of Radiobuttonlist's as they care to use and have "Finaltxt.txt value" updated every time a new button is pressed. - I also imagine they may keep the browser open all day and use it perodicaly.

Question 2:

I am not really sure that I should be doing "Maths" from data held in a text box. (I would have assumed this data to be a string, however it does work, I would have thought I would need to convert it in some way but I can't find an appropriate command/syntax.

Question 3:

How do I format the value shown in "Finaltxt.txt value", I would like this figure to be rounded up to two decimal places.

I would appreciate any help given.

Best wishes.

James.perhaps u should have a command button to do the actual conversion