Class 12-science

You need minimum 55% to take admission in government college for aviation courses. And here, 5 marks relaxation for career willing SC/ST students is given.

In between the time, you must be giving entrance exams for various colleges, but if still you have time and want to learn something you can start learning programming language may it be C, C +, python or Java that you can decide according to your familiarity with the skill, if you are a beginner go for C. As nowdays in Science community programming is a must have skill may it be any department you and learning programming languages are quite easy as there are so many MOOC course available over the internet.
All the best.

Bro, it is based on your interest. In Science stream, there are many options like B.E, B.Tech, BBA, B.Sc, BCA, etc. You may gain your skills and knowledge, then you gets better job opportunities in your future. If you are interested in business field, then go to BBA after you go for MBA. If you have interests in computer field, then go for B.E. (CS/IT), B.Tech etc.
Best wishes.

For getting admissions in B.Tech or any other UG programme. Ongoing dates are very important to fill the form. Every college has it's own procedure and time and date to start the application process. You must check on the NET. JEE main application has been closed already. You may take the private colleges like Amity University, SRM University, Galgotia University etc. The Galgotia University will be having it's UG programme test on 15th June, hence you have a chance to fill it. Best of luck
For more query, please feel free to ask.

Try going for a Diploma in graphics, and then pursue Bachelor's degree may be. In this way, your time won't get wasted if you didn't qualify. And try again.

Please refer to the link provided below for details regarding a Graduate Diploma in Management from Indian School of Business Management and Administration, Kolkata. You should be able to download college brochure which contains guidelines regarding admissions to this course, and eligibility criteria. Alternatively, you can visit college official web site for further details regarding this course.
https://www.shiksha.com/bba/course/graduate-diploma-in-management-indian-school-of-business-management-administration-kolkata-jawaharlal-nehru-road-107149
(Website: https://www.isbm.org.in)
Regards.

There are numerous options available for you after completing class 12. Please refer to the link provided below where you can access these options, and select an appropriate course after class 12th (Science). https://www.shiksha.com/careers.

As your brother is studying in class 12th with CS, it will be beneficial for him to study CS further, but obviously, he needs to be interested in it or even he can do another course which is not related to CS like doing B.Sc in the subject which he likes or whichever course he likes in engineering or even he can choose a different field of his interest but the bottom line is if he does what he likes it will be beneficial for him and for his family.

If there is nothing mentioned about individual subjects then you just need to have a combined percentage of three greater than 60% to become eligible.
All the best.

All depends on your effort if you study well it won't be hard for you. As you said that you had CS in class 12th, that will come handy as you will be a level ahead of most in computers.
All the best and choose wisely.