If you are in class 11th so go through well with its mathmatics syllabus as it will help you for NDA preparation. Here are some preparation tips you can follow:

Prepare mathematics with the help of the basics of class 9,10,11,12. If you are done with the basics start practicing from a good book.  Be consistent with your preparation.

General Ability Test consist of two sections-English and General Awareness. The English section can be tackled by practicing questions such as error finding, fill in the blanks, Jumbled para, reading Comprehension etc.

For general awareness you need to study the social studies of class 9th and 10th and current affairs. You need to read newspapers for the current affairs.

The weightage of the general awareness section is very high so you need to be more focused on current affairs.

Join mock tests to analyze your preparation.

Good Luck!