Official series and sessions are explained in the video below.
There are many ways to join an official series.
From the iRacing UI (aka desktop icon):
Click the 3 horizontal lines to toggle the Sidebar to open and close
Click Go Racing
Under that you can choose either Up Next, Current Series, User-Created Races, AI races or Time Attack
In Up Next or Current Series, you can Filter to show what you want to see
and in the results, you can sort by any of the columns, for example, License Group
then click the green Register button for the race (or session) you want to get into
Follow the online instructions and then click Join when prompted:
