Hii aspirant..

You don't need to register yourself to IIM for getting admission there. If you are sitting in CAT exam and have qualified their eligibility criteria and cut off percentile then you will get call from the IIM itself for further admission procedure.

Also there are some top 20 schools which you should register for before your CAT exam. Some of the top P School you need to register are -

• FACULTY OF MANAGEMENT STUDIES (FMS) NEW DELHI
• SHAILESH J MEHTA SCHOOL OF MANAGEMENT (IIT BOMBAY) MUMBAI
• JAMNALAL BAJAJ INSTITUTE OF MANAGEMENT STUDIES (JBIMS) MUMBAI
• DEPARTMENT OF MANAGEMENT STUDIES (IIT DELHI) NEW DELHI
• MDI
• SP Jain
• IMT Ghaziabad

Hope this helps. 

Best of luck