Hi, Manideep, you can wear a shirt and jeans for your Jee Mains exam there is no problem with it and you can also wear T-shirts also. My suggestion is that looking decent and wearing some decent clothes will be good instead of being funky and remember you are going for an exam, not for a fashion show. 

This year National testing agency is conducting the JEE main exam for the first time and national testing agency has not released any dress code for JEE main exam but a formal dress code holds more important for JEE main.last year CBSE conducted the exam and which allow only light sleeves and loose clothes for the exam.