Dear Sheetal ,

For 12th standard , you can study NCERT e book of computer science , which is available here :

https://ncert.nic.in/textbook.php?lecs1=0-13

Well, you can also use reference books for individual chapters like :

• data Structure- Reema Tehreja
• Python book- Tata McGrew Hill
• Let us C - Kantekar etc.

The e-book version of the above books you can find through google.

Hope this helps !