NATA

The National Aptitude Test in Architecture (NATA) measures the aptitude of the applicant for specific field of study, I.e. Architecture. The test measures drawing and observation skills, sense of proportion, aesthetic sensitivity and critical thinking ability that have been acquired over a long period of time, and that are related to specific field of study, I.e. Architecture.
I would suggest you to sketch a lot and train your hands and mind, to sketch better every time you start to sketch. Other than that you need study a few aptitude questions about general knowledge and basic architecture, which is available in a few books/guides meant for NATA.

NATA stands for National Aptitude Test in Architecture. It is an entrance examination which measures the aptitude of the applicant for study of Architecture. Any person who has passed 10th standard or equivalent examination and above are eligible to appear in NATA examination. But, to get admission in B.Arch. course through NATA examination, you must have qualified an aptitude test in architecture and must have secured at least 50% marks in your 12th standard in your 12th standard or equivalent examinations with Mathematics as one of the subjects from a recognised board. Since, you have completed 12th standard from a recognised board, you are eligible to appear in NATA examination.

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.