Hello Amit,
You have to apply to JOSAA after JEE MAINS to get admission in NITs, IITs, IIITs and GFTIs.
And you also you have to fill the counseling form of some of the colleges like BITS Pilani, VIT Vellore, etc..

Hello Amit,
You able to fill application form for B.TECH after April Jee main.
As after April Jee main,NTA will release rank of a candidate. NTA will also release category cutoff.
After this many institutions will release department Cutoffs for various categories student.
Wait till end  of April .
The best of both Jee will count for making final ranking.

If you have any questions remain than comment below.
Regards...

Hi Amit Gupta., 

Can you clear tell me ,what the application form is about, and which college, 

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