Aviation

Your Mechanical degree will be an additional qualification for you, although it will not be used for any privileges for issuing your CPL.
For becoming a commercial pilot, there is a specific eligibility criteria, which you should first be aware of. This is decided on what subjects have you taken in class 12th. It requires 50% in PCM to be a pilot. And even if you have not opted for the same, then you can appear separately for these subjects and become eligible.
Further, you should be aware that you are medically fit to be a pilot, so I would suggest you to visit http://www.dgca.nic.in/medical/medical-ind.htm and go to the Class 2nd examiners tab to find out doctors who will examine you for initial fitness. You can visit any doctor from the list as per your convenience.
Once you are done with Class 2nd medical, find out a suitable flying club for you as per your convenience. You can fly in India or abroad. I would suggest you fly in India. Besides finding the club, be aware that you will have to appear for:
1. Air Navigation
2. Aviation Meteorology
3. Air Regulations
4. Aviation Technical
5. Aircraft Specific
6. Radio Telephony
Here, 1-5 are conducted by DGCA and 6 is conducted by WPC. Prepare for these subjects and appear, as exams are conducted every quarterly. For preparation, flying club will help you or there are private instructors available in metro cities as well.
You will have to fly for 200 hours+ clear DGCA and WPC exams + clear Class 1st medical, then you shall get your CPL.
The next license is ATPL, which you shall be able to apply for at once.
You will then have to probably fund or self-sponsor your type rating on whatever fleet you are selected in an airline. Then, you finally get a job as a pilot.
I hope the process is well explained to you. CPL is an expensive affair. It might cost you around Rs. 18-20 Lacs for flying + DGCA exams Rs. 500 each exam + medicals for Rs.10,000 + endorsements and license issue fees around Rs. 25k + type rating around Rs. 15 Lacs. So, decide wisely before you enter the field.