NAT or National Aptitude Test is a common entrance test for admission to different programmes offered by participating institutions. This year 54 institutions, including three universities, are participating in the NAT testing system. Conducted by Society for Research & Development in Education (SRDE), Delhi (www.srde.org), NAT also tests the aptitude of the candidates.
The test is conducted at the Under Graduate (NAT-UG) and the Post Graduate (NAT-PG) level. NAT-UG is a common test conducted at the national level for admission to various Bachelor level programmes i.e. BBA, BCA, B.Sc., B.Tech, BSc.(MLT), BA, BFT, B.Com, BPT, B.Ed, BMC, B.Lib.Sc., B,Pharma etc. of various participating institutions as per NAT Information Bulletin. The basic eligibility is 10+2.
NAT-PG is a common test conducted at the national level for admission to various Post Graduate programmes i.e. MBA, MCA, M.Sc., M.Tech, MA, M.Com, MFT, MMC, MIB, M.Lib.Sc., PGDCA, PGDBM, PGDBA, MCM, MMM, MPM, PGDFT, PGDHMNA, PGDHM, PGDT, PGDB, LLM, MPT, M.Pharma etc. of various participating institutions as per NAT Information Bulletin. Eligibility for appearing in NAT-PG is graduation.
The test is an objective type multiple-choice answer using the Optical Mark Reader (OMR) technology which has already been vetted by leading Government/private Institutions of the country. Duration of the test is two and a half hours. Sample papers are provided on the website. The test is held every year in May/ June at many places all over India.
The NAT scores are valid for one academic session. SRDE sends the NAT scores to five institutes of the candidate's choice. However, applicant can request SRDE for supply scores to additional institutions by submitting a formal request in the prescribed proforma.
Please keep in mind, that mere taking NAT does not qualify a candidate for admission to a university/institute's programme. Each participating institution has different weight age for NAT score in their admission criterion of different programmes. Thus interested candidates may also be required to participate in other admission activity of the institute like group discussion and /or interview etc.

I was interested in aeronautics and astronomy, but I have now realised that it would immensely tax me. Should I change my goal or stick to it?

Don't underestimate yourself, you can do anything, if you're really interested in it. But first you must find out, what it is that you really want. Was it only a fascination with the moon, stars and planets that made you feel like you are interested in astronomy?
If you're not sure about this, give yourself some time. Find out about your interests, abilities and the subjects that appeal to you. Choose only that stream in 11th, which has the combination of most of your favourite subjects. Astronomy is a specialised field of Physics and is offered only at postgraduate level. If you like science subjects then you can decide about taking up astronomy after graduation in Physics. But if you don't like science, then you've already decided not to go for astronomy.
But make your decisions based on your abilities and capabilities, because your interest alone can make you work harder and help you go to any heights.

What qualifications are required to become a school teacher? Please tell me more about NET and SET examinations.

B.Ed. (Bachelor of Education) is the requirement for teaching in secondary schools. Graduate teachers who are B.Ed. are known as Trained Graduate Teachers (TGTs). This is a one year course and can be done after graduation with any stream. But for higher secondary teaching, you should be a post graduate in the related subject, along with B.Ed. This one year course is offered by most of the universities throughout India. Graduates with a certificate or diploma in education can qualify for primary school teaching. Due to increase in number of schools, there is demand for good teachers. National Eligibility Test (NET) and State Level Eligibility Test (SLET) are the tests conducted for those who are interested in teaching at college and university level. Those who have cleared the NET are exempted from appearing in the state-level test SLET. Before appearing in these tests, you must be a post graduate in the subject you're interested in teaching.

I am studying in Std XI. My subjects are Psychology, Sociology, Philosophy, Politics, English and German. Please provide some information on the future scope of Arts.

Combined with your personal interests and talents, education in arts subjects can be a valuable preparation for a variety of career paths and for leadership and management roles in many fields. Though a degree in arts like B.A. does not directly lead to a job, arts graduates are found to be flexible, well-informed individuals with excellent communication skills. With this degree, you can appear in many competitive examinations e.g. for IAS, armed forces, banks or even get a job in call centers, travel and tourism companies etc.
Otherwise, you can go for professional courses in the fields like mass communication, advertising, law, management, design, library science, insurance, secretarial practices etc., after completing 10+2. But if you are particularly interested in some subject, you can go for further specialization in that subject. For example, psychology is recommended if you have interest in working with people. A strong foundation in the understanding of human behaviour opens the career door to a variety of options including personnel work for a company, child development, counseling, and law, just to name a few.
So do not worry about the future scope, rather concentrate on your present studies and do well.