CBSE 12th

Hi Anshul,
Even though the exam to be undertaken is popularly known as IAS exam, it is officially called UPSC civil services exam. The UPSC CSE consists of 3 stages: Prelims, Mains, and Interview. Getting into Indian Administrative Service is not easy considering the competition involved, but not impossible for a candidate with right attitude and approach.
This exam is widely considered as the toughest exam in the world, considering the exam duration (extends 1 year), depth of the syllabus and the competition involved. The first stage of the exam Preliminary (objective) is usually conducted around June, while the second stage, Mains (written), is conducted around October. Those who clear Mains will have to face interview/personality test in the March-May period. The final result usually comes in June.
A disciplined and focused mind with a passion for learning and reading is the basic trait of all successful candidates. The UPSC Civil Services exam is more a test of the analytics and presentation skills along with knowledge gained by the candidate rather than a test which checks candidates mathematical or verbal skills.
If you start as early as class 12th, then you should concentrate more on the static syllabus and complete it comprehensively and in depth. Plus, pick up the habit of reading the newspaper regularly, which help you in understanding current affairs and various socio-political affairs.
Some of the important books are:
History of Modern India by Bipan Chandra (History)
Indias Struggle for Independence by Bipan Chandra (History)
History of Medieval India by Satish Chandra (History)
Indian Art and Culture by Nitin Singhania (Culture)
Geography of India by Majid Husain (Geography)
Oxford School Atlas Oxford (Geography)
Indian Polity for Civil Services Examinations by M. Laxmikanth (Polity)
Indian Economy by Ramesh Singh (Economy) or Mrunal Video Lectures
Economic Survey (Economy)
Science and Technology: Follow current affairs
Environmental: Shankar IAS Environment Book
India Year Book (Current Affairs)
Manorama Yearbook (Current Affairs)
For CSAT:
CSAT Paper 2 Manual by TMH or CSAT-II Arihant (CSAT Paper 2)
Analytical Reasoning M. K. Pandey (CSAT Paper 2: Analytical Reasoning)
Verbal & Non-Verbal Reasoning RS Aggarwal (CSAT Paper 2: Reasoning

You have many options in front of you if you want to pursue writing after high school. One of these options is a course in journalism which would mean you would have a license to practice journalism. You could also take up writing courses like creative writing from symbiosis or diploma in writing from IGNOU. You could also become a certified content writer and enter the industry with viable certifications.
Apart from all these options, you could also publish technical journals which are highly paid in the industry, however you require a great amount of expertise to write journals. You could start by writing blogs, articles to gain some practice and a foothold in the industry. You could start by writing for yourself, and graduate to writing as a professional.

Eligibility Criteria for Graduate Course (Bachelor of Architecture-B.Arch) are:
Most of the Architecture colleges in India offer education in Architecture at undergraduate level and students with mathematics at +2 level are preferred. Duration of the graduate course is 5 years.
1.Minimum 50% marks (45% aggregate for Reserved category) in 10+2 Examination or equivalent with Mathematics as subjects, or
2) 10+3 Diploma (any stream) recognised by Central/ State Governments with 50% Aggregate marks, or
3) International Baccalaureate Diploma, after 10 years of schooling, with not less than 50% marks in aggregate and with mathematics as compulsory subject of examination.