I guess you are here because of google and his "website speed test": https://developers.google.com/speed/pagespeed/insights/
No worries, I will help you. There are various options over the net about how to achieve this.

The easiest way is the html5 way.

Let's say your javascripts code is:

<script type="text/javascript" src="/common/js/jquery.min.js"></script>
    <script type="text/javascript" src="/common/js/jquery.lazyload.js"></script>
    <script type="text/javascript" src="/common/js/tooltip.js"></script>

You will have to modify it as follows:

<script type="text/javascript" [color=green]async[/color] src="/common/js/jquery.min.js"></script>
    <script type="text/javascript" [color=green]async[/color] src="/common/js/jquery.lazyload.js"></script>
    <script type="text/javascript" [color=green]async[/color] src="/common/js/tooltip.js"></script>

FYI, there is also the classic method which you can use:

<script>
  var resource = document.createElement('script'); 
  resource.src = "[color=green]/common/js/jquery.lazyload.js[/color]";
  var script = document.getElementsByTagName('script')[0];
  script.parentNode.insertBefore(resource, script);
</script>

Or a fancy defer:

http://www.feedthebot.com/pagespeed/defer-loading-javascript.html

No matter the method, be careful, some of your website content (like menus, for example) do not like asynchronous load so you will have to leave them as is or load them directly in the code:

<script language="javascript">
        // common/js/jquery.min.js
        the famous code....
        </script>