JEE Main

It all depends, like if you do not have 75% marks 12th, then in order for being eligible for admission through JEE, you have to take the improvement exam. But JEE will accept that scorecard only when you have given the improvement in all the 5 subjects.
For exact details visit- www.eduncle.com/iit-jee
Since the JEE doesn't want to look at your two different report cards and calculating the percentage on the basis of them.
So, if you have scored more than 75%, then there is no need for giving the improvement in all the 5 subjects, as you are already eligible for JEE exam.

No brother, you are not eligible to appear in IIT JEE 2018. To get all the details about IIT JEE eligibility
Visit- www.eduncle.com/iit-jee
As per the JEE Mains 2018 eligibility criteria, a candidate is eligible to appear in IIT JEE for a maximum 3 times, that also in consecutive years.
IIT JEE 2017 was the last chance for you to appear in IIT JEE exam.

No JEE Mains cut-off score must be qualified to get admission in IIT as well as NIT.

No, you can't. According to 2016 cut offs of AIT, Pune for CSE branch is 68239.
According to JEE Mains marks vs rank, to get this rank you need to score 130-140 at least. But cut offs varies every year. So do as much as you can.

BITSAT, VIT, Manipal University Entrance exam, SRM, COMEDK are some of the good ones to apply for other than JEE.

IITs give affirmations just on the premise of the JEE Advanced positions (AIRs). The following are not utilized:
JEE Main scores, e.g., IIIT-H use these scores.
All India Ranks touched base at through the standardization system utilized by JEE Mains. These positions are utilized by NITs to give confirmation.
Board checks just fill in as qualifying criteria for admissions to the IITs. Fundamentally, this implies regardless of the possibility that you get an average AIR in the JEE Advanced, you would not be considered for confirmation unless your Board marks are over a specific rate or percentile.

It depends upon the category and cut-off of JEE Mains examination. But a minimum of 180 marks are required by a general category candidate.

NIT-Trichy takes admission on the basis of JEE Mains paper-1 given by B.Tech aspirants.

Hello friend,
The official notification is not released for IIT JEE 2018. But there are few changes in the exam criteria.
For example, there will be no effect of the 12th board exam on the final score of IIT JEE. Previously, 12th marks had some weightage included in the final IIT JEE marks. So from 2018, this is not going to happen in IIT JEE.
Hope this will help.
Best of luck!

Yes, you will get admission, if your JEE Mains score is good, otherwise you will get average rated college.