If you are starting the competition preparation, you can start with the AOPS introductory books and move to intermediate/advanced books. At some point you can start with competition preparation books. Take the tests in the beginning of the competition series books ( https://artofproblemsolving.com/store/item/aops-vol1, https://artofproblemsolving.com/store/item/aops-vol2, https://artofproblemsolving.com/store/item/competition-math ) and start solving problems from these books. Also start practicing with level1 problems from previous year handbooks or OPLET.

Different options to practice are available on the following links in this website

https://sites.google.com/view/omos-math-club/mathcounts-links-details

Start with Introductory books from AOPS

https://artofproblemsolving.com/store/item/prealgebra

https://artofproblemsolving.com/store/item/intro-counting

https://artofproblemsolving.com/store/item/intro-geometry

https://artofproblemsolving.com/store/item/intro-number-theory

Move to Intermediate/Advanced Series

https://artofproblemsolving.com/store/item/intermediate-algebra

https://artofproblemsolving.com/store/item/intermediate-counting

https://artofproblemsolving.com/store/item/precalculus

Start using competition series books once you are ready. It could be even before taking the intermediate/Advanced series

https://artofproblemsolving.com/store/item/aops-vol1

https://artofproblemsolving.com/store/item/aops-vol2

https://artofproblemsolving.com/store/item/competition-math

https://artofproblemsolving.com/store/item/mc-practice

https://artofproblemsolving.com/store/item/mc-practice2

Also start practicing with AMC-8 problems

https://artofproblemsolving.com/wiki/index.php/AMC_8_Problems_and_Solutions

If you feel comfortable with AMC-8 problems, move to AMC-10 and AMC-12 problems

https://artofproblemsolving.com/wiki/index.php/AMC_10_Problems_and_Solutions

https://artofproblemsolving.com/wiki/index.php/AMC_12_Problems_and_Solutions