To get admission in NIT Agartala in B.Tech program, you will have to crack Jee Main exam as this is the only way to get seat over there and if you still haven't apply for Jee main then the application window is open till 1st Jan so you can fill the form.

To check how to apply for Jee and how much rank is required to get admission in NITs, you can go through the below links -

Jee Main 2018 Application Form

Jee Main cut off

Good Luck!!!