I passed class 12 this year 2023 and I am trying to prepare for CAT now. What should I start with first?

Few tips for you - 1) Focus on getting good grades (aim for 85%+) 2) Try to participate in co-curricular activities (don't be just a book worm) 3) Build a habit of reading (read what you like) 4) Build a habit of solving puzzles (play chess, sudoku etc.) 5) Start your prep only when you enter your last year of graduation (burn out is a real thing!)

Agreed with all the tips given by Nihaal except the last one.

No you need not to start from the first year. Start your prep on the year you'll be appearing for cat . 6 to 10 months of prep is considered to be an ample amount of getting into IIM . However, you can start reading novel nd newspaper from this year .