Setting up your rFactor 2 server is very different compared to other games.
1) Find the MAS2.exe inside rFactor2/Support/Tools and open it.
2) Click on the box as shown in the image below:

3) Click "Create New Mod Package":

4) Enter the Mod Name. (see image below) Keep in mind this Title will show in the server browser, make it an informative title (which cars are used for example)
5) Click on Next >>

6) Select the tracks you want to run on your game server (select multiple by pushing the CTRL key)
7) Click on Next >>

8) Select the Vehicles/Cars.
9) Click on Next >>

10) Add Sound. This is important as mods often have external download links so joiners can install via the server. if you do not add Sound, they will have no sound.
The rest of them, AI/UI/etc is less important.

11) Click on "Package"

It should now show you "Mod Packaging complete with no errors" (see screenshot below)