Try this at the top:
<button type="button" class="navbar-toggle" data-toggle="collapse" data-target=".navbar-collapse">
<span class="icon-bar"></span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
</button>
Also, add this at the bottom of your page before the closing tag
<script src="//code.jquery.com/jquery-1.10.2.min.js"></script>
<script src="js/bootstrap.min.js"></script>