Try a challenge and practice your skills! Create a pull request to submit your attempt. View others attempts to learn how to tackle a problem from different angles.
To make a pull request hit the "Pull request" tab between Issues and Wiki. Once you are in the tab hit the big green button in the top right of the page that says "New pull request". Here you are able to compare the main branch to your branch. Comment and submit a pull request for review!
Each code challenge is organized into it's own folder. These folders contain a markdown file explaining the challenge. When creating a markdown file, have it named with an "_" in the beginning. This will move the file to the top of the folder. There are folders for different languages. If you don't see an answer in your language yet then feel free to create it! Save your file name with your GitHub username and save to the appropriate folder.
These questions come from a variety of sources include online examples, class work, Cracking The Coding Interview book and interviews.