Code Forces is a site where competitive programmers can practice. Most of the problems in this site will be too difficult for High School students to solve. However, the site segments its problems with a difficulty rating between 800 and 3000. Problems with difficulty ratings between 800 and 900 are well suited for High School.
When getting started with Code Forces site, it is not cheating to look at the tutorials that accompany some of the problems. You need to familiarize yourself with the problem types (spatial geometry, combinatorics, etc.), some of which you may have never seen before. By studying the tutorials, you can train yourself methodically to tackle those types of problems. Eventually, you want to get good enough to solve similar types of problems without any help.
The first video (below) features on of the worlds' top programmers describing how to get started in competitive programming. Below that are two videos which walk you through (step-by-step) two of the easier problems in Parallel Projection 1781A and Code Forces: Matrix Rotation 1772B. Links are provided below. You should try to do these problems on your own before you look at these videos.
Video to help you get started with CodeForces
We suggest you start your CodeForces experience by working on problems with a difficulty rating 800. If you find these too easy, you can slowly move up to 900 and 1000 and then higher.
# Problem Difficulty Rating
String Task 800
Watermelon 800
Puzzles 900