You need to load fabric.js first at the header and then you need to load your js file order matters.
<script type='text/javascript' src='//cdnjs.cloudflare.com/ajax/libs/fabric.js/1.4.0/fabric.min.js'></script>
<script type='text/javascript' src='yourjsfile.js'></script
In case you are not clear on how to add elements to the canvas. Then here is one snippet which could help you.
Also wrap your code inside onload() function so that when the script loads, the #draw element is available
window.onload = function() {
var canvas = new fabric.Canvas('draw');
canvas.isDrawingMode = true;
canvas.freeDrawingBrush.width = 5;
canvas.freeDrawingBrush.color = "#ff0000";
// create a rectangle object
var rect = new fabric.Rect({
left: 100,
top: 100,
fill: 'red',
width: 20,
height: 20
});
// you need to add the object to canvas
canvas.add(rect);
}