Hii,

JEE mains is a common entrance test to get admission in NIT's and IIIT's.If you arr qualified in the JEE mains then you can write JEE advanced which is for admission to IIT's.To write the JEE advanced you need yo get more than 85 percentile. As you got 50 percentile you can't write that exam.But some private colleges take admission based on the JEE score.But you won't get good colleges with this percentile.

Thank you