Greetings aspirant!

To get into IIMs the first step is to give the entrance exam called CAT. Other factors that are responsible for admission into IIMs are WAT/GD-PI performance, academic history, academic diversity, gender diversity and work experience.

Stages of IIM selection process.

• cat exam
  
• selection on the basis of cat exam score
  
• Selected candidates apply for WAT/GD-PI
  
• Screening of candidates for WAT/GD-PI
  
• Last stage is calculating candidates composite score and analysis of other parameters.
  

Also as per your question no you cannot pursue IIM just after your 12th you will have to complete your graduation. Also to know about CAT syllabus click the link below.

https://bschool.careers360.com/articles/cat-syllabus

hope you find it helpful.
thankyou!