Government & Defence Entrance Exam

UPSC Prelims Syllabus for GS Paper (Prelims Paper I)
Current events of national and international importance. History of India and Indian National Movement. Indian and World Geography Physical, Social, Economic Geography of India and the World. Indian Polity and Governance Constitution, Political System, Panchayati Raj, Public Policy, Rights Issues, etc. Economic and Social Development Sustainable Development, Poverty, Inclusion, Demographics, Social Sector initiatives, etc. General issues on Environmental Ecology, Biodiversity and Climate Change that do not require subject specialization General Science
Syllabus for CSAT Paper (Prelims Paper II)
Comprehension
Interpersonal skills including communication skills
Logical reasoning and analytical ability
Decision-making and problem solving
General mental ability
Basic numeracy (numbers and their relations, orders of magnitude, etc. ). (Class X level), Data interpretation (charts, graphs, tables, data sufficiency etc. Class X level).

It is not possible to become an IAS Officer right after completing 12th standard.
In order to become an IAS Officer, you must apply for CSE exam conducted by the UPSC. You must also crack the exam (preliminary, Mains and interview) in order to get selected for the training. The basic eligibility criteria required to appear for this exam is candidate must possess Graduate Degree from a recognized University/Institute.
So technically, class 12th passed students can't appear for this exam right after class 12th. After completing class 12th, they must complete Graduation first. After completing Graduation programme successfully, they may take a shot and CSE and become an IAS Officer subsequently.