What is the Procedure to become an I.A.S. officer after 12th???? which entrance exam do I need to crack????????

Sorry you can not become an I.A.S. officer after 12th, the basic requirement to apply for the IAS examination is graduation and then you need to appear in the Civil Services Examination which is conducted by UPSC every year for the recruitment of candidates for the post of Indian Administrative Service.

Nationality:
A candidate must be a citizen of:
A citizen of India, or
A subject of Bhutan, or
A subject of Nepal, or
A Tibetan refugee who came over to India before 1st January, 1962 with the intention of permanently settling in India, or
a person of Indian origin who has migrated from Pakistan, Sri Lanka, Burma, East African countries of Kenya, Uganda, the United Republic of Tanzania, Zambia, Malawi, Zaire, Ethiopia and Vietnam with the intention of permanently settling in India.

Required qualification:
The applicant should hold Degree of any of Universities incorporated by an Act of the Central or State Legislature in India

Age Limits
Candidate must have attained the age of 21 years and must not have attained the age of 30 years on 1st August

This is the IAS exam syllabus:

General English
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)
English Language Comprehension skills (Class X level)
Essay

General Studies
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
Economic and Social Development-Sustainable Development, Poverty, Inclusion, Demographics, Social Sector Initiatives, etc.
General issues on Environmental ecology, Bio-diversity and Climate Change – that do not require subject specialization
General Science.