how should i prepare and time table for preparing

Marks you strong areas and divide time according to it. But yes, study regularly.

TIPS FOR NDA PREPARATION:
Fix your goal and start preparing early, as early as after your Class X exam. This will give you an edge over other students.
Understand the syllabus thoroughly and make a firm routine so that you cover all the subjects
Go back to your basics, even if that means referring to your Class VI, VII, VIII, books subjects.
Improve your English. Start reading a newspaper daily and make it a regular habit to watch news on TV. Also, brush up your Grammar skills.
Refer to the past years question papers so that you have a fair idea of what to expect.
Solve as many mock question papers and past question papers as possible.
Identify your weak subjects and start working on them so that you gain a stronghold.
Prepare yourself to be calm in the examination hall and answer the questions within the stipulated time.
As there would be negative marking, try to prepare yourself in such a manner that you do not have to make any guesses in the examination hall. It is best to answer only if you are very sure of the answer. Guesses are not going to pay here.
Spend time on understanding yourself, your weak areas and try to strengthen them.
Start thinking and behaving like an officer.