After 12th you need to first enroll yourself for a graduation program as per your interest, as it is the minimum qualification for IAS preparation. After that

You will have to crack the Civil Services Examination conducted by the UPSC . It is conducted in 3 stages

1. Prelims
2. Mains
3. Interview

For its syllabus & exam pattern you can go through with given below link.

IAS syllabus