Hello Abhay, according to the eligibility criteria of JEE Main 2019, you will be eligible if you qualify the Senior Secondary Examination conducted by National Institute of Open schooling with a minimum of 5 subjects in the year 2019, or have cleared it already in 2018. 
It is compulsory to take at least 5 subjects in class 12 or any other equivalent exam. These subjects for B.Tech and B.E are:
1) Language
2) Physics
3) Mathematics
4) Choose any one of the following: Chemistry, Biology, Biotechnology, Technical Vocational Subject.
5) Any other subjects
These 5 subjects for B.Arch and B.Planning are:
1) Language
2) Physics
3) Mathematics (compulsory)
4) Choose any one ( Chemistry, Biology, Biotechnology, Technical Vocational Subject)
5) Any other subjects
Please note that,
The following eligibility will be considered during admission but not during the exams:

• For admission into NITs/ IITs / CFTIs:
1) Candidate should have scored 75% marks or secured a position in the top 20 percentile in his/ her 12 exam or any other equivalent qualifying exam.
2) And for SC/ST category this eligibility criteria is 65% in the 10+2 examination or any other equivalent exam.

• For admission to B.Arch programmes:
1) It is mandatory for the candidates to have PCM combination in 10+2 exam
2) They must score 50% aggregate marks in all subjects and 50% marks individually as well.

Age limit: There no age limit to apply for JEE Main 2019. Only you should have passed class 12 or should be appearing this year.
Number of attempts: A candidate can appear in the JEE Main exam only for three consecutive years. Even if the candidate chooses to attempt the entrance twice in a year, the attempt for a particular academic year will be considered as one.

If a candidate has fulfilled all the criteria except the percentage required for taking admission into IITs, NITs and CFITs, then a candidate can go for improvement test.
Also, in case you are going for improvement test in class 12 in 2019, there are certain provisions:
Improvement in one or more subjects: so if a candidate is appearing for improvement test in 2019, then the top 20 percentile of 2018 will be considered. And the percentage will be calculated on the basis of maximum marks obtained in the required subjects.
Improvement for all subjects: if the candidate appears for improvement in 2019, then best of both the years will be considered.

For further information regarding the eligibility of a candidate, please visit: https://www.google.com/amp/s/engineering.careers360.com/articles/jee-main-eligibility-criteria/amp
All the best!
I hope this helps.