Someone please guide me for IOAA (most likely NSEA, i.e., the first stage of IOAA in India), like what material and books to use for mathematics, physics and astronomy and is it too late to start my preparation in the 11th class?
I've bagged a gold medal for India in the IOAA-Jr, so I think I may answer your question to some extent.
Math and Physics are pretty standard introductory JEE level stuff they ask in NSEA. If you take out some standard JEE book, you might as well be done doing just till the medium level exercises. Important concepts like gravitation, magnetism, mechanics should be done at a bit more depth.
They don't ask too much hardcore astronomy in the first stage. Just learn some basic definitions from 'Astronomy: Principles and Practice, A.E. Roy, D.Clarke' like stellar magnitudes, Lagrange points. This book is literally god for astro olympiad aspirants. Look up the chapter on celestial coordinates. Don't delve into the math part of it now, only understand the basic terms, and voila! You're good to go for NSEA! You can look up some constellations and learn spherical trig too if you wanna be on the safer side (but that's more relevant only for INAO). Anyways, love Astronomy and keep working hard. I wish you the best of luck!