If you want to join a private league which a "commissioner" has already created, you must get the league name and password from the commissioner. After you enter your details, you will be taken to the league home page, although your commissioner will still need to formally approve your team before your draft starts.
Public League
Public leagues require a fee paid by each coach, depending on how long the competition lasts for. When you apply to join a public league and make your payment, you are placed in a queue while the transaction is processed. When 9 other coaches are with you in the queue and your transaction has been approved, a public league is created for you automatically. You will receive an email notifying you of the exact date and time of your draft, which will take place on the day following the one on which your league was created. All public leagues have the same settings, which are detailed in the league tutorial and will be listed on the Rules page in your League menu.