All extensions are free or I have to pay a money for that. I am creating a extension then how will I get the money?
In the CiviCRM extension directory all extensions are free. It is common in open source to share your work for free.
I am a freelance CiviCRM developer and all my extensions are free. However I get paid by clients for developing extensions for them and I then share those extensions for free.
So one way is to find clients and start working for them.
May I ask what brings you to asking this question? Is it that you like developing and you what some rewardds or is it something else?