You write an HTML form, and you write a server-side HTML form handler that sends email. Basically, you need to learn a server-side web technology (language and libraries) well enough to handle HTML forms and send emails from it.
There are a lot of technologies you can choose for the server-side handling of HTML forms (PHP et. al). An important factor in your decision will be what the hosting providers available to you support. All else being equal, though, I would pick
Flask.