IIT

No, IITs do not provide any commerce related courses. They only offer B.Tech, M.Tech, MBA and Ph.D. programs.

75% marks is minimum to get IIT.

What you have to do is check out whether you are ready to face IIT or NIT entrance exams, if answer is yes, then go for it. Choose next IIT classes in your city give the entrance exam of that institute. Coaching is the only way to prepare yourself for IIT. So, Be ready to face challenges. Study hard

When you are in different branch other than computer Science in engineering, you can study Computer Science side by side as per your interest not going for the syllabus. You can learn programming languages by yourself, there are lots of online classes available and you can go for coaching centers al

Hello Yogesh, I am afraid 120 marks in JEE Advanced are rather low for getting admission in an IIT.

There is no role of class 12th percentage for admission in NITs and IITs only a minimum of 75% is necessary. So, if you have got 75% in class 12th, then there is no need to drop for board instead of this you should focus on JEE Main and Advance exam.

First of all, congratulations for your excellent result.
Your rank will be near 25000. You probably cannot get seat in IIT, but off course you have good option for NIT College. You can see the list of NIT and IIIT College which you can get:
https://www.shiksha.com/b-tech/resources/jee-advanced-rank-p

You can apply for JEE main only. If you clear the JEE main exam, then also you would not be eligible for appearing the JEE Advanced because for it, you should have 75% aggregate marks in class 12th. Best of luck.
Thank You.

Many coaching are present for JEE Mains and IITs. If you want best coaching, then you have to do self-study with last year sample papers of Arihant book.