Hello Abhishek,

Your basics in Physics, Chemistry & Mathematics should be very clear upto 12th std. You should read the NCERT books for basics. Then HC Verma for Physics, RD Sharma for Mathematics, OP Tendon for Physical & Inorganic Chemistry, and Organic Chemistry of "Arihant Publication" for detail or extra

Yes you have to pay fee at IIT the tuition fee at it is 1 lakh rupees per semester and other pays including hostel rent and mess advance sums up to 20000 per month.

SC ST students are given full waiver in tuition fee apart from that students are divided

Yes you shall be eligible for Jee Advanced if you appear for 12th in 2019 year and then you can get two successive permissible attempts in year 2019 amd 2020.

You must have PCM as mandatory subjects in 12th grade from recognised University and have atleast 75%aggregate marks in 12th

Fees at IITs and NITs vary across different institutes. For IITs, the Tuition fee is fixed at INR 2,00,000 per year i.e. for Two semesters excluding Mess fee and Hostel rent and other Gymkhana fee which is different for different IITs and it is between 20,000 to 50,000 per year

Marks above 250 is must for ranks ubder 200 in General category as per 2018 statistics. 

128 marks is minimum under General category for qualifying as per 2017 trends.

Students can get all rank estimates below.

https://engineering.careers360.com/articles/jee-advanced-rank-list

Hi

If you are applying in IISER by appearing in aptitude test conducted only for SCB channel candidates, there will be no interview. You just have to qualify in the test. 

As far as I know, you will have to appear in all the four subjects in the aptitude test

Admission to IIT is based on Jee Advanced marks and to be eligible for Jee Advanced you need to qualify from 2.31lakh candidates based on Jeemain scores. 

Cutoffs or minimum marks for all IIT is different based on streams, Category and quota. Cutoff list for all IIT shared below. 

https://engineering.careers360.com/articles/jee-advanced-cutoff