Go to View | Toolbox. Grab a TextBox control, and drag it into the page. Then repeat this action with another TextBox control. Next, beneath that, place a Button control, and then under that, place a Label control so that you have a place to display some output.
This is the basic user interface. Let's change a couple of things here, though. First of all, put the <br /> tags to insert line breaks so that the markups are on separate lines. The Label control doesn't need one because there's nothing underneath it, so it can stay there as-is. Now change the text on the Button tag so that it says, for example, Summarize. Your Default.aspx page should appear as the following screenshot:
Next, we will read some values, store them to an array, and then, for example, show the total of the...