Hello,

For admission in any college such as IIT, NIT, GFTIs or IIITs, the admission process is simply through performance in JEE. After which JoSAA allocates seat in such institutes. Having a Diploma from any ITI,etc would be beneficial only if you want to do jobs. Private colleges such as VIT, KIIT, etc too have their own entrance examinations. So start preparing for these entrance examinations. Best of Luck! 

You'll have to appear for various entrance examinations to get into the top colleges as the colleges but if you have done your diploma from the same University where you are planning to pursue your btech from then they'll let you start from 2nd year onwards as you'll be considered as a lateral entry student but then also they require a decent score in diploma. But certain colleges are there where you can get direct admission in 2nd year even if you don't fall among the top students or are from different University. Few colleges in West Bengal University of Technology accept that.