Hello, 

These are the some preparations tips :

•  You should try to fix your goal and start your preparation as early as possible.
• You should first understand the NDA syllabus and should make a routine to cover all the subjects.
• You should not forget to update the basics of each subject. And  refer to the books of Class VI to XII.
• To improve the English, you must start reading a newspaper on a daily basis, also brush up your grammar regularly even you should also learn the usage of the new words.
•  refer to the previous year’s question papers and  should solve mock question papers to have an idea of the exam.
• You  must first spot your weak subjects and start working on them to improve in that particular subject.

Hope this helped.

Good luck!