The process of sending a mail happens server-side, HTML/JavaScript is client-side. As far as I can see you're not using a web-server nor are you hosting the website somewhere.
There is a FakeSendMail option with the new install from XAMPP which you could use to emulate the mail() function from PHP. XAMPP is one of the most known localhost web servers , which you could use to finish your project, in case you don't really need that mail to be actually sent. If you do, I recommend using a webhosting.
But first you need to understand the difference between Client-Side and Server-Side. As far as Client Side is concerned, all it does is render your static data. (HTML/CSS/JS). But, as for Server Side, there is a lot more use to it, as you can work with a database, fetch and insert data from or to it, and eventually render data which will be processed by the Browser (client side)