As you have mentioned that you are in 12th std and of 18 yrs old, then you can surely appear on NDA 2 exam.
As per its eligibility criteria, candidate must born between 2nd Jan 1999 to 1st July 2002.Also, candidate should have completed/appeared in 12th board exam.
UPSC will notify on 7th June 2017 regarding this exam. Application form can be submitted between 7th June 2017 to 30th June 2017. Candidate will get admit card before one or two weeks of exam. Exam will be conducted on 10th Sep 2017. Result will be declared on Oct 2017.
You have sufficient time for preparing .
Have good luck.