Javascript/HTML- Displaying Name Input From Form

I'm trying to figure out how to get javascript to display values entered into a form element.

I may be doing this wrong from the outset and this is not the standard way to do it but here is what I'm working with.

HTML

<form>
            First name: <input type= "text" id="firstname" name="firstname"> <br> <br>
            Last name: <input type= "text" id="lastname" name="lastname" <br> <br> <br>
            <input type = "submit" value="submit">

            </form>

JavaScript

<script>

    function getElementById("firstname"){
        var userName= document.getElementById("firstname")

            document.write("Hello, " + userName);


            </script>