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:
From the Members website (members.iracing.com) using a web browser (Google Chrome, Firefox or Edge):
One way is to click the green Race Now tab at the top right of the members Home Page
Choose the series from the drop down on the left side, and also choose a type of Session at the top right (Race, Qual, TT or Practice).
Another way, from the members website, is to click Find Official Races
You can filter whether you want to see Oval, Road, Dirt Oval, Dirt road sessions, Fixed only, etc. Make sure to click the SAVE button each time you make a change to these filters.
Then click the green steering wheel for the race you want to get into.
A third way, also from the members website, is to hover your mouse over Find Official Races and choose Current Series
Here you can filter by Catetory (Oval, Road, Dirt Oval, Dirt Road, Nascar, Multiclass) and License Level.
Also, Expand All will show you more details for each series (and Collapse All closes that information).
Click SELECT for the series you are interested in and this will automatically populate the Race Now tab where you can then join the Race