As it depends upon you.
If you can do it your own than there is no coaching required.NDA consists of maths, gk, English and some basic knowledge.
For mathematics you can refer NCERT books and you would need a good practice .
For GK and general awareness you can refer daily newspaper reading and news channels and a good book of general knowledge . So if you are good at maths you can prepare yourself.
After passing the exam , the main stage is ssb in which your skills, ability , communication skills ,English proficiency and talking and living skills will be observed.
So good luck
NDA exam is so designed that you don't need any specific coaching for it.
1. It has two papers one is maths and other is general studies.
2. Maths paper is from 11th and 12th and thus you don't need any coaching for this paper.
3. For general studies just go and solve arihant book which will be sufficient as most of the questions will be from basics.
Check the link