In this age of internet, resources are abundant. This can be confusing for many students.
Make sure to choose 1 book and complete
all problems from it from cover to cover. Do not browse thousands of websites, solutions and books in search of the perfect resource. There isn't one.
For middle school students start with Mathematical Circles by Fomin. High School students may focus on Challenges and Thrills of Pre-College Mathematics.
Read about other recommended books as well.