IIT

You have to qualify the JEE Mains and Advanced entrance exam and also qualify HSC board exam with 75%.

All branch choices depend on your interest and all branches are good if you have a good knowledge about it.

You may refer to the following link for tips on clearing JEE: https://www.shiksha.com/b-tech/exams/jee-mains/preparation-tips

Total seats in IITs are 11 (4 in IIT Powai and 7 in IIT Guwahati ) for B.Design for SC category students. You have lesser probability of getting admission in IITs for B.Design.

Check out the eligibility predictor for JEE Advance :
https://www.resonance.ac.in/answer-key-solutions/JEE-Main/2017/jee-main-2017-eligibility-predictor.php

Go through books of IIT-JEE books and previous year question papers.

If you are diploma candidates then you have to start your engineering with first year for IITs by qualifying the JEE Mains and Advance exam.

If you need to take admission in IIT, you need to qualify for JEE Mains paper 1.

It is just great that you have thought to study B.Arch at IIT.
You should appear for JEE main paper 2 (B.Arch).
Almost all the coaching centers are good. I don't want to mention any coaching center names. If you want to get into any best institute then refer those which produced the toppers in JEE exams this year or the consecutive years.