I have a website on Joomla platform and I am new to programming world..I want to know cautionary steps that I should take before releasing my site .

Use the most current version of the software and keep it up to date is the best recommendation I can give you.

There is no way to absolutely guarantee the safety of your site, but maintain a current version and update when the version changes. Especially when they issue 'security' updates

On top of jlhaslip's suggestions, take care with your login information. Don't choose a username like admin or administrator. If you have something that simple, any hacker is already half-way in! Then focus on a good strong password. Make it alphanumeric, use upper and lower case, throw a few symbols in there, and even put a space in if you want (sometimes this isn't possible).

Also,you might want someone you trust to try your website for security holes. If they find any, they should be able to tell you how to sort them out.

The most important part is to be very carefull with input from the visitor, always check it. All the $_GET's the $_POST's make sure they won't be able to use bugs in your code to get in. Because 80% of all sites can be hacked because of bugs in their scripts.

take a look at the following functions:

htmlspecialchars();
is_numeric(); // very helpfull with $_GET['id'];

also take care of the include methods and upload and file management systems. So they can't fake files, by changing their extentions and so on. Lots of articles about it on the net. Please watch them

You should also back up your files and database constantly. There is a feature in the CPanel that lets you back up all your files. I have a modification for my forum that stores a copy of the database in a selected directory daily, as well as daily emails of a back-up. This way the most that can possibly be lost is 1 day. But my forum is IPB. Im not sure about Joomla.