This first thing you'll need to decide is where your logic is going to sit, Client or Server Side.
Client Side - Would mean that the calculations are contained in the HTML, this would mean you would be using JavaScript to do the calculations. You would have to layout your form and when the user click a "Calculate" button then you execute a piece of Javascript code (contained in your HTML) that picks the values from your page, performs the calculations and displays your result.
Server Side - Would probably mean using something like php (provided you host supports it). You would layout your form and when the user click "Calculate" the form would be submitted to a .php file which would perform the calculations and return a page display the result.
Client side is probably going to be the easiest way for you to go. It will mean you have less to learn, but it probably won't be as flexible/powerful as if you went server side.
Layout your page and then look at JavaScript functions like document.getElementById and math functions to help you interrogate the page and display results.
HTH,
JC