In order to take admission in CSE you have to appear for entrance exam of engineering.

To get admission in IITs and NITs you have to appear for JEE (Main+Advanced). In order to be eligible you should have 75% marks in your boards exam or you should be in top 20 percentile. 

For SC/ST it is 65% marks. 

To take admission in top private engineering colleges you can appear in BITSAT, VITEEE, SRM, Manipal and Amrita. 

These colleges provide good knowledge, exposure and placements. 

You can also appear for your state level entrance exam to take admission in state government colleges.

Good Luck!!