First of all, congrats on being selected.
JEE maths and Engineering maths are a little bit different. You'll find that out when you reach here. Till then, just chill out.
Though it helps if you are good at JEE maths, you can still score well if you put efforts right from day 1. Most of the professors are very approachable. You'll be having tutorials in addition to classes. You'll be allotted Teaching Assistants who will help you out with problems. A little effort from your side and you'll do great!