This whole page is written out by my classmate Gaurav(APhO Gold, IPhO Silver) who has much better experience of Olympiad Physics than me. I hope you would find this helpful!
Useful Resources for Starting Out
The knowledge required for physics olympiads is very similar to the introductory physics syllabus at university. This is great, because it means you can take advantage of some of the truly excellent resources that are available for undergraduate physics.
Yale’s Fundamentals of Physics courses, taught by Professor Ramamurti Shankar are wonderful. Prof. Shankar is also very funny! You can find YouTube playlists of the lectures on the YaleCourses channel: here is a playlist for the first of the two courses, which introduces the basic principles of mechanics, waves, and thermodynamics. The video lectures for MIT’s 8.01 and 8.02 courses are another great resource, filled with interesting experiments.
A standard textbook is Fundamentals of Physics by Halliday, Resnick, and Walker. A more advanced version is written by Halliday, Resnick, and Krane. These textbooks are great introductions, and they are both really common: you will probably encounter them as you study physics in one way or another. To get a deep understanding of Newtonian mechanics (which is essential), Introduction to Classical Mechanics by Morin is really useful.
You shouldn’t worry too much about which resources you decide to use, as long as they help you explore the key ideas behind mechanics, waves, thermodynamics, and electrodynamics (and later, special relativity too). When watching video lectures, remember to pause often and think about what has been said, and to work through derivations yourself. The same applies to textbooks: try not to skim too much or rush through topics. Take the time to appreciate the beauty of the physics you study: that its laws can be written down so simply, that these laws lead to such a wealth of theory and concepts, and that the theory actually describes our world.
Textbooks and online courses are a great source of problems to reinforce your knowledge as you learn the fundamentals. JEE preparation will also help significantly with the mathematics and physics concepts.
It’s also a good idea to start working on competition problems as soon as you can. A couple of good sources for mechanics problems are the Hong Kong Physics Olympiad and the USA F=ma exam. And past problems for the competition you are preparing for, such as INPhO, are of course a good source.
For more challenging problems, some nice sources are the USA Physics Olympiad (USAPhO) and the Pan Pearl River Delta Physics Olympiad. You can find translations of many other national Olympiads here. And of course, there are the International Physics Olympiads: APhO, IPhO, and EuPhO. There is a nice set of recommended problems from Olympiads, made by Jaan Kalda.
At https://www.ioc.ee/~kalda/ipho/ you can find several handouts, a formula sheet for IPhO, and many other resources made by Jaan Kalda. At https://physoly.tech/ you can find solution manuals for Kalda’s handouts and many other resources, as well as a link to a Discord server with thousands of other high school students preparing for physics competitions.
For the resources listed here, it’s really important to have a good understanding of algebra, pre-calculus (such as trigonometry), and calculus. If you’re looking for more advice/resources for starting out (such as physics textbooks that do not require calculus), take a look at https://knzhou.github.io/writing/Advice.pdf, which helped me a lot in writing this.
If you want to gain a deeper understanding of certain topics or go beyond Olympiads, there are a bunch of great resources listed at https://knzhou.github.io/writing/AdviceAfter.pdf.