You have enough time. Focus on your 11th and meanwhile start with the basics.

Start with having a smart study plan which means managing your time and NDA syllabus efficiently. Do the analysis of your complete syllabus and mark the topics that require more concentration. Only when you have good basics then you can go for next difficult questions/level.

Your fluency in English is tested and so practice it from the grammar. GK is one of the other important section. You can strong your GK portion by reading newspapers, magazines, journals, refer books of prominent writers that focus on current affairs, etc.

You should go through previous year papers to know the level of question expected. Selection through NDA is based not only on the written test but your personality and health also plays a major role in it. You can check NDA physical standards here.

You can prepare from books:

• NCERT books of 11th and 12th class.
• Pathfinder for NDA by Arihant
• Maths from RD Sharma
• NDA/NA (National Defence Academy/Naval Academy) Examination Guide: Entrance Examination
• NDA and NA Entrance Examination-Previous Solved Papers

Good Luck!!