hello aspirant,   To become a civil servant that is there are totally 3 services INDIAN CENTRAL AND STATE . This popular civil service category of IAS comes under category group A - which comprises 24 services like IAS IFS IPS IRS .. are top most indian services . To become an IAS officer or any other cadre of group A one has to appear for Civil service examination conducted by union public service commission .This exam is conducted in three phases

1. PRELIMINARY - CONSISTING OF TWO OBJECTIVE TYPE PAPERS(GENERAL STUDIES PAPER 1AND 2(popularly know as civil service aptitute test CSAT)

2.Main examination consisting of nine papers of coventional essay type, in which two papers are qualifying and remaining 7 papers are counted.

3. These two phases are followed by INTERVIEW (PERSONALITY TEST)

If you qualify in all the three phases you will be a civil servant and your choice of your service depends on your rank , that is , the more you are ranked the more there are chances of you selecting your most preferred cadre.

all the best.

Hope it helped.