How do I become a commercial pilot?

I have some questions in this regard:

1. Can I get a CPL by completing 250 hours on any aircraft such as the cessna 172 or a particular aircraft is required?

2. After getting my CPL can I join an airline as a FO before getting an ATPL?

3. What kind of course will I require to convert from the cessna 172 to an airliner such as the 737? And what is the average fees for such a course?

4. What else will I require after getting my CPL to fly as a FO for an airline?

1. No. You require AT LEAST 10 hours of instruction in an aircraft of 200hp or more that has retractable landing gear, a controllable propeller, and wing flaps, otherwise known as a high-performance, complex aircraft. The C172 doesn't check that box. You'll also need a multi-engine rating with instrument flying privileges.

2. No. Not in the USA which requires an ATPL and a minimum of 1200 hours, and not in any other country in the world unless you have very powerful friends in very high places. 250 hour pilots do not get copilot jobs with the airlines, nor do the paying public or the insurance companies want such inexperienced pilots in that position, not even with a small regional airline.

3. There are many courses you need, including high altitude endorsement, CRM training and a type rating. \You'll also need several years of experience as a commercial pilot flying in a non-airline position. Depending on where you live, that could take a lot of years because "general aviation" jobs are not plentiful. Unless you were to graduate from an airline cadet training program, NOBODY goes straight to the airlines after finishing their CPL ratings.

4) If you are living in India, as your name suggests, there's very little chance of easily getting a job because there are far more inexperienced flight school graduates than there are positions available. If you're a resident of India, there's a lot of competition for jobs and a fresh flight school graduate has very few opportunities to get any type of flying job that will give them the experience to qualify for an airline. True fact. It's the law od supply-and-demand.

1) At SOME point, you're going to need to log hours in a "High performance" aircraft.
2) NO. You don't "JOIN" an airline, you apply and get HIRED by an airline, and NOBODY is going to HIRE you with just 250 hours.
3) You don't go directly from a Cessna 172 to a 737, Skippy… You need a bit of experience in other, more complex aircraft first, and it costs more than you have in your piggy bank.
4) A Bachelor's degree from a REAL college or university and about 10 years of commercial flying experience.