IIT

Students with Diploma in engineering recognized by AICTE or a state board of technical education of at least 3 year duration are eligible to appear for JEE to seek admission in IITs.

Generally, JEE Advanced exams are conducted in the month of June.

You need to qualify JEE Advanced exam in order to pursue B.Tech in aeronautical engineering in IIT.

To get a good NIT or IIT, you need to score above 220 marks in JEE Mains out of 360 marks.

Syllabus of IIT JEE and BITS is same but in BITS you have to prepare for English. Also, the level of BITS is higher than that of JEE Mains and 12th board but low in comparison to JEE Advanced. I think you should first focus on JEE Advanced so can you will be able to solve good quality questions and able to clear both BITS as well as JEE Advanced
Best of luck.

Hello,
If you want to get selected in an IIT, then you need to appear for GATE or IIT JAM exam.
IIT JAM is for M. Sc, Ph.D, and other PG courses.
Check out the link metioned below:
http://www.slideshare.net/PriyankaGupta1234567/how-to-crack-iit-jam-entrance-test-for-msc-tips

If you are determined well to crack IIT or NIIT, drop a year and prepare well.

Devoting two full years for competitive exam preparation has its own merits and demerits. It is worth try option if you want to become an engineer from IIT only. If you succeed then you do not lose much but failure will lead to two productive years wastage / loss.