Students who are appearing for the Class XII exams from a recognized board or university are also eligible to apply.The NDA I 2018 exams are over. The registration for the NDA II exams (conducted by UPSC) are likely to be announced in June-July 2018. The exams are likely to be around 9th Sep 2018.
The results are expected to be available by end of October 2018. After this would be two stages of Service Selection Board Interviews. If you get selected, then You would need to produce the Certificate at the time of document verification. For the NDA-I 2018, the date to produce the certificate is 1st December 2018.
If you look at NDA-2 2017, the exams were in Sep 17, Results in Nov 17, the SSB dates were in Jan 2018 and the final merit list is expected in June 2018. The time gap between the NDA written exam and the SSB interview is anywhere between 3-4 months. From your post, you say that you are becoming XII student in June this year. This would give you an idea, please check the UPSC official site and get the clarification.