With 45 marks in JEE Mains 2018 in the OBC category, you are definitely eligible for sitting in JEE Advanced exam and I assume you must have already registered yourself for JEE Advanced examination whose registration's last date is today (7th May 2018) and exam will be conducted on 20th May 2018.

After that in order to be eligible for getting admission in IITs, you need to clear the cut-off of JEE Advanced examination. Last year JEE Advanced cut-off for OBC category student was 115 marks. Even after getting marks near to cut-off you will only be eligible to participate in the admission process of getting a seat in IITs. As 50,000 students are made eligible for this but there are only around 10,000 seats so in order to confirm your seat in any of the IIT you need to aim for scoring way more higher marks. If you don't qualify the JEE Advanced exam cut-off then you won't be eligible for any process.

Feel free to ask if you have any more queries. All the best!!

If you have selected in mains exam and eligible for giving jee advance and second thing is that one need to clear exam with good rank.Jee advance is very tough exam ,and it need very hard work .