To be able to take part in the ranked mode, you need to complete the first two chapters in the story mode. This means, you need to complete 10 battles. Seasons can vary in duration so make sure you take part as soon as it starts. You can set up your defence for the battles on the left side by clicking on a character and choosing another character if you want to change the setup.
By clicking on the info button next to “ranking” you will see the current ranking and which league you are a part of by clicking on “league/season”. There you will also be able to see the points you receive, your level-up rewards and the season rewards.
By clicking on the info button next to “reports” you can see which player attacked when, from which MC and how many times they have won.