NATA

Any person having passed SSC (standard X) or equivalent examination and above can appear for NATA. However, only candidates who have qualified an aptitude test in Architecture and have secured 50% marks in 10+2 or equivalent examination with mathematics as one of the subjects, shall be eligible for admission to B.Arch course. No direct/lateral admission is allowed at any year/stage of B.Arch course based on any qualification. NATA score shall be valid for a period of two years from the year in which one appeared.
You need to submit your NATA scores to the local admission authority for a government seat or approach the college directly for the college quota seat.

Sumit, chances of your name appearing in further rounds depends upon luck. If some students surrender their seats, then only your name can appear in further round.
Better would be try for admission in good private universities. These days we have so many good private universities are also imparting quality education to the students around the world. They offer you admission on JEE scores/ tests conducted by them/ NATA scores? your marks in 12th. Some of these also offer good scholarships based on any one of the above mentioned admission criteria.
Check the websites of some good universities or call on their toll free no.s for more details.
Good luck.