Dim prompt, sideLength As String
Dim area, length As Double
prompt = InputBox("Enter sides of the square.")
' On return from this method, "prompt" will contain the value typed by the user
' so that is the string that needs to be converted to a number.
sideLength = Convert.ToDouble(length)
' You are trying to convert the variable "length" to a number,
' but you have not done anything to get a value into it.
' You have also declared "sideLength" as a string, but now
' you are trying to store a Double value into it.
area = length * length
Me.lblOutput.Text = "Square of side length " & length & ": " & area
Take a look at http://msdn.microsoft.com/en-GB/library/6z0ak68w(v=vs.90).aspx[^] for an example of how to use the InputBox method.