To prepare for the Sainik School entrance exam (AISSEE), you need to study subjects like Maths, English or your regional language, General Knowledge, and Intelligence (reasoning).

If you are preparing for Class 6, focus more on basic maths, reading, and general knowledge like science, India, sports, etc.
If you are preparing for Class 9, you should study Maths, English, General Science, Social Studies, and reasoning questions.

Some good books to use are the AISSEE guides from Arihant or MTG. Solving previous year question papers is also very helpful. You can also read your NCERT books to understand the basics clearly.

Try to practice maths every day, read for 30 minutes, and solve at least one model paper every week.

Here are some useful websites where you can find study material and practice papers for the Sainik School entrance exam:

Arihant and MTG have books that help you prepare for all subjects. You can buy them online.
You can also download previous year question papers in PDF format from education websites like aglasem and examrace.
These sites also give tips, solved papers, and mock tests to practice online.