You can start with the course of Olympic Mathematics, which is not very difficult.

There is also "High School Mathematics Competition Training Textbook", which is simpler than "Olympic Mathematics Course" written by Li.

The first two books are basically oriented to the national high school mathematics league.

The Orsay Classic is more difficult for me.

There is also a "math competition research class", which is also very difficult.

Are you satisfied with the above answers?