I did not get above 75% in 1st year boards can I write mains?

To qualify for JEE Mains, there is no minimum aggregate criteria. But if you want to join NITs, IIITs or CFTIs then you should score a minimum aggregate of 75% only in your 12th. 11th marks aren't considered. And then to appear for JEE Advanced which is the gateway for admission to IITs, you will need to fulfill the 75% aggregate condition. Now it is 75% for General Category and 65% for Reserved Categories. So how much you score in 12th is what matters to secure your eligibility for NITs, IIITs, CFTIs and JEE Advanced. So if you did not score 85% in your 11th or Intermediate First year, leave it at there and move on to 2nd year and ensure you score 65-75% based on your Category. Your target should be 75% for the 2nd year. This is easy if you stick to NCERT revise the syllabus the same day that it has been taught in the class and practise as much as  you can. If you can strategise a timetable and follow it strictly, even by studying 1 hour a subject, you can easily score 75% in your 12th boards. And that way it is easier for you to qualify for NITs, IIITs, IITs and CFTIs. Good luck:)

There is no percentage barrier to be eligible for Jee Main. With any percentage you can appear for Jee Main. However, for admission to IITs, NITs, IIITs, CFTIs boards marks are taken into consideration. For these institutes, you should have secured 75% ( 10% relaxation for reserved and pwd candidates) in aggregate or must be in top 20 percentile of your board, and you match this criteria. So nothing to worry about. Read our article http://engineering.careers360.com/articles/jee-main-eligibility-criteria