Diploma students can apply in IITs, They need to appear for JEE Main Entrance exam first, after qualify the exam they need to applyfor JEE Advance Entrance Exam, On the basis of JEE Advance score they can get admission in IITs in the first year of B.tech programme, You can not get admission in NITs.

If you are interested to take admission direct second year of B.tech programme, You can go for State level or University level Entrance Exam.

Good Luck!