Registration for the season opens in May.
To start a team you need to create a DASHBOARD account. Sign-up for a dashboard account here: https://my.firstinspires.org/AccountManager/Account/Register
The FIRST website also provides a plan for starting a team https://www.firstinspires.org/robotics/fll/start-a-team
Registration usually closes around mid-September.
Robotics kits may also be purchased from Lego Education or with a slight discount through your Dashboard once your team is registered