You can debug your program by printing debug statements to STDOUT. For questions that need you to write the full program your debug output will appear in the section called 'Your Output'. In such cases, remove all your debug statements before submitting your solution, as the entire output will be matched with the expected output and your program likely will not pass the test cases. For questions that expect you to complete a function, all your print statements will be captured separately in an area called 'Debug Output', and they will be ignored while comparing your output with expected output.
Many problems also allow you to give you an option to 'Test against custom input': you can use this option to specify your own input values to run your test against. When you select this checkbox no other test cases will be executed.