NIT

You have to register online. Visit the website and check for registration link.

The CAT entrance exam is a national entrance exam acceptable in India only.
For CAT coaching in Mumbai - refer https://www.growthcentre.org/cat-coaching-classes/
The GMAT entrance exam is accepted by universities in the USA, UK, Australia, New Zealand, Canada, Singapore and many European countries.
For GMAT coaching - refer to https://www.growthcentre.org/gmat-coaching-classes/
Obviously, GMAT has more weightage because it is internationally recognised by thousands of universities in the world.

Hi,
For getting admission into CSE branch in top 5 NITs, you need to score very well for that, so any score more than 200 is considered safe for getting admission.

Yes, you can expect but not all NIT's. You have a fair amount of chances, don't worry you will get admission in a good NIT.

Hi,
To get admission into NITs, IIITs and GFTIs under JEE counselling you need to have at least 75% (for general category), 65% (for SC, ST and PWD candidates) or be in the top 20 percentile from your board (in the top 20% students from your board).

Here are the NITs which provide aeronautical engineering branch:
National Institute of Technology, Tiruchirappalli
NIT,Warangal
NIT, Trichy
NIT Suratkal, Karnataka
NIT, Rourkela
NIT, Kurukshetra
Neither of the IITs and BITS have aeronautical in their courses.

No, if you are doing diploma in mechanical engineering you cannot apply for IIT or NIT because it has stopped taking any lateral entry admission and if you have done your 12th then you can surely apply for the JEE mains advanced.

Hello Sai Kumari,
As per your marks in JEE Mains, your chances are nil for getting any NITs, however, check here to know more.
https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor

Hello Vivek,
The cut off marks vary every year for each category but it is required to have a score like 250 and above to get a top NIT for SC category. You need to have a score like 100 and above to get an NIT.