How to save form data into a download-able text file

Question

 #maintitle
    {
        text-align: center;
    }
    #asterik
    {
        color: red;
    }
    #background
    {
        background-color:  rgb(235, 229, 222);
        font-family: 'Quicksand', sans-serif;
    }
    #firstname
    {
        font-size: larger; 
        background-color:  rgb(235, 229, 220);
        font-family: 'Quicksand', sans-serif;
    }
    #lastname
    {
        font-size: larger;
         background-color:  rgb(235, 229, 220);
         font-family: 'Quicksand', sans-serif;
    }
    #dropdown-list{
        background-color: rgb(235, 229, 222); font-size: x-large;
    }
    #phoneno
    {
        font-size: larger; background-color: rgb(235, 229, 220); font-family: 'Quicksand', sans-serif;
    }
    #Email
    {
        font-size: larger; background-color: rgb(235, 229, 220); font-family: 'Quicksand', sans-serif;
    }
    #submitbutton
    {
        font-size: larger; background-color: rgb(235, 229, 220); font-family: 'Quicksand', sans-serif;
    }

<form id="form">
 <h1 id="maintitle">Application Form</h1>
 <hr>
 <hr>
 <br>
 <h2 style="font-size: larger;">Name (Important)</h2>
 <hr>
 <input type="text" id="firstname" placeholder="First Name" >
 <br>
 <input type="text" id="lastname" placeholder="Last Name">
 <br>
 <hr>
 <h2 style="font-size: larger;">Phone Number (Important)</h3>
 <hr>
 <select id="dropdown-list">
    <option>+91</option>
    <option>+1</option>
    <option>+86</option>
    <option>+33</option>
    <option>+49</option>
 </select>
 <input type="text" id="phoneno" placeholder="Phone Number">
 <br>
 <hr>
 <h3>Email (Important)</h3>
 <hr>
 <input type="email" id="Email" placeholder="Put your Email here.....">
 <hr>
 <button type="submit" id="submitbutton">Save / Submit</button>
</form>

I'm a very young programmer just learning front-end development and don't know alot. I have written this code and want it to save when button is clicked :

I want that when the Button or the thing with id 'submitbutton' is clicked, the form saves the data filled in the form and transfers it to a text file which is downloaded automatically when the button is pressed.