To draw a line in the canvas that you need to insert the following code in your <script></script> tags:

<script type="text/javascript">
  var c = document.getElementById("canvasTest");
  var canvasElement = c.getContext("2d");
  canvasElement.moveTo(0,0);
  canvasElement.lineTo(100,100);
  canvasElement.stroke();
</script>

Here, canvasElement.moveTo(0,0); is used to have our line start from the (0,0) co-ordinate of our canvas. The canvasElement.lineTo(100,100); statement is used to make the line diagonal. The canvasElement.stroke(); statement is used to make the line visible. I would suggest you to change the numbers in canvasElement.lineTo(100,100); and canvasElement.moveTo(0,0); and see the changes to your line drawn by canvas.

The following is the output of the code: