As a newbie, I am quite confused how open source software is can/be licensed. I know of licenses like GNU public, Apache, and others.
What is the procedure to license a open source software by developer? What considerations should be made? What choices do I have?
Technically most open source software aren't 'licenced'. You don't buy a licence to use it. You just use it. The "GNU General Public Licence" is not really a software licence that you accept or decline, it's a copyright licence
Books are copyrighted, you can't go photocopying a whole book, and sell it yourself. However you don't 'buy' book licences. Open Source software is like that.
There's no big scary procedure. :) All you have to do is drop a file named LICENSE with your license of choice inside of it into your project's root directory and make it obvious that that file is the license for the entire project, either through comments in the source code or a notice in the README.
Choose any license you want. A quick google search can get you on your way.
One of the most commonly used ones in open source is the GPL.
As the developer you are free to license your software (provided you aren't developing the software for someone else) in whatever way you wish. You have all the choice in the world and you should choose a license that meets your needs. Beyond that, any advice is rhetoric.