I am familiar with the front-end development. I can create a website but I can't able to add behaviour to it. I mean I am not getting how to implement javascript into that.
You can add javascript to your website using the <script> tag.
For example, here's a simple "hello world" script:
<script>
alert('Hello world!');
</script>
You can place this tag anywhere in your document. Usually it's at the bottom of your body tag.