That ROI calculator that you posted an example to is actually made out of Flash rather than java/javascript. Here's the direct link to it: Clicky

So if you wanted to try and clone something like that, you'd need to do it with Adobe Flash, but then you're back to square one, with trying to learn a whole new scripting language (such is the life of a programmer). If you know the formulae, you can do it in pretty much any language; Flash, Javascript, Java, PHP, any .NET flavor, old ASP, etc.

The language you use is also slightly dependent on the server Hosting the site you'll be showing it on, and the way you choose to code it also depends on how you want to display the results. i.e. Do you want it to show in realtime on the same page (like the Flash example)? Or do you want to have a new page show after the data is entered?

Basically the idea is to make a normal .htm page, add a form to it with all of the fields you need the person to enter their input on, and a submit button (if not doing it in realtime). The submit button can start the calculations and spit out the results to the user.

All of that said, if you have no experience doing all of this, and it all looks Greek to you right now, and you're in a time crunch to get it done, you might want to try and bribe a coder (it's not real hard so it doesn't' need to be a super-genius coder) somehow, and let him/her do it for ya.