How to fill in form field, and submit, using javascript?

If I have an html document whose rough structure is

<body class="bodyclass" id="bodyid">
<div class="headerstuff">..stuff...</div>
<div class = "body">
<form action="" id="login_form" method="post">
<div class="form_section">You can login here</div>
<div class="form_section">
<input xmlns="" class="text" id="username"
       name="session[username_or_email]" tabindex="1" type="text" value="" />
<div class="form_section">etc</div>
<div xmlns="" class="buttons">
    <button type="submit" class="" name="" id="go" tabindex="3">Go</button>
    <button type="submit" class="" name="cancel" 
            id="cancel" tabindex="4">Cancel</button>

You can see that there is a username field and a Go button. How would I, using Javascript, fill in the username and press Go…?

I would prefer to use plain JS, rather than a library like jQuery.