Career

You could try banking or accountancy, depending on your area of interest.
Even call centers are a good option if you are good in communicating.

If you are serious, you should join one of the good professional colleges that provides training & grooming and qualify you for an air-hostess.
However, I would suggest you to complete your graduation first.
All the best.

Career after 12th science is very vast.
You can opt for engineering, medical, management, teaching, research related courses.
Even you can go for civil services and state government services.
For a student of science stream, it's easy to jump into commerce and arts field. Like, you can opt for BBA or mass communication or engineering.

Cracking civil services will become easy if you have knowledge of each of the subjects. UPSC never demand genius of any particular subjects.
Here, I am suggesting you some tips to start your preparations:-
1. Reading national newspaper daily and cut and paste important news this will be your notes.
2. Read magazines like India today and Pratiyogita Darpan.
3. Watch news daily, especially DD news .Watch all debates with guests of any news channel.
Watch documentaries of news channels.
4. Refer books of good writers.
5. Don't go for quantity (hours) of studies rather go for quality of your study.

I am a mechanical engineering student. Which specialisation should I opt in MBA to increase my career opportunities and salary prospects?

Management programs are not exactly in programming but you can get into position with a job profile managing IT but for that, you have to get your MBA from an institute that ranks in top 100 B-Schools in India.

Along with psychology, history is a better option to attempt in civil. Because it works two way, one as a subject and second, it supports GK to score in civil examinations.

You may look at any part time course in your nearby locality.

At this stage, I would strongly recommend you to do your post-graduation in MCA as alone B.SC (IT) won't let you go far in your career as per current market stats. When you complete your MCA, you will be considered as professional and this will open doors of opportunities. And yes, doing certifications only add skills. But being professional is very important.
I hope it helps.

Yes, it is credible and have a lot of scope.