At first you need to apply for the entrance examinations. Different colleges have different admission processes. For IITs and NITs you need to apply for JEE Main examination. For the preparation of JEE main you can refer to the link below:

https://www.entrance360.com/knockout-jee-main-april/

For other colleges there will specific entrance examinations conducted by that particular colleges itself. You can register to these examination from the college site itself according to your wish.

Also some college entrance is done through the state's entrance exam portal. You can find that according to the state that you are in.

Hope this helps. For more details on which college you want admission please ask .