To become an IAS officer you have to follow the examination procedure laid down by UPSC as given below:
Firstly, apply online/offline for UPSC Civil Services Exam held every year starting in the month of May. (Make sure you fullfil all the eligibility conditions before applying for the exam)
Secondly, Take Civil Services Prilimanry Exam (Top students will be eligible for taking final exam probably first 10,000).
Thirdly, If you are able to clear Civil Services Prilimanry Exam, take Civil Services Main Exam. (Top Students will be called for Personal Interview nearly Ten times the number of Post)
Finally, Once done with Main Exam you will be called by UPSC for Personality Test and Personal interview.