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!)

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 .

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