Most leagues in rFactor 2 require your real name. You can find all available settings in 'Singleplayer' > 'Options'. You can tweak most, but not all, of the settings during a session (whilst in the garage). If that doesn't solve the problem, delete the corresponding files in '.\SteamApps\common\rFactor 2\Packages' & '.\SteamApps\common\rFactor 2\Installed'. If a mod causes a problem, try to uninstall/delete it first.
Not all mods have to be installed, some are automatically downloaded when you join a server.
For all other mods, you will have to manually check if they have been installed - if not, mark them and click install, on the bottom left. If you downloaded your mod through Steam Workshop, it is very likely that it will be installed automatically.