I'm making a program which calculates a user's BMI (Body Mass Index). I am fairly new to VB.NET so please bare with me.

This is what I have so far...

Code:
Public Class BMI
    Inherits System.Windows.Forms.Form
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        'an integer is one or a group of characters has a value ranging from 0 to 255
        Dim number1 As Decimal 'user enters height
        Dim number2 As Integer 'user enters weight
        Dim answer As Decimal 'signals that the answer must be a decimal

        'Val returns the numbers contained in a string as a numeric value of appropriate type
        number1 = Val(TextBox1.Text) 'number 1 equals TextBox1
        number2 = Val(TextBox2.Text) 'number 2 equals TextBox2

        answer = number2 / (number1 * 2) 'this shows what the calculation is and both number1 and number2 will be added together

        MsgBox(answer) 'a message box will appear with the user's BMI

    End Sub

End Class
The concept works absolutely fine, however, I want to extend it a bit.

I'd like to include some sort of validation, so the user is permitted only to use numbers and not letters. So if they did enter a letter or having said that, if they enter anything in the wrong format, an error message will appear.

I'd also like the pop-up box with the user's BMI to say whether they are under, over, or a healthy weight.

Could anybody help me with this?