Welcome Bart!
Besides the Golden Rules and the posted League Regulations, I don't think there is a single forum for beginners at the moment. As it is, though, you have found the best way to get help here, and that is to post a question. Tons of friendly folks here to help you out with just about anything.
In general, I would start with a few club races to get settled in. If you have the RACE series, the American Racing Club runs nightly races, with each night of the week dedicated to a different type of car. The European Racing Club runs multiple daily races, and includes GT Legends, rFactor and other simulations, though you will find most of the weekday races scheduled for around noon-ish mountain time.
Personally, I always download the tracks and any car mods as soon as I see a race I might be interested in (before signing up) - they are posted in the first post of a race announcement as Senad said. That way I can get the feel of whether or not I'm going to like the car, and can 'sight' the track. After I've convinced myself (whether or not it's true) that I can handle the car/track combination, I'll go ahead and sign up. You can post any questions about the event itself in the related thread. Once signed up, I will usually wait until the day/night of the race to download any skins/liveries, to make sure I get any late posted ones in.
Once you're signed up, you are good to go. If something comes up and you will not be able to make a race you've signed up for, please post letting the race director know you won't be able to make it, as some events fill up and that will allow room for the first reserve into the race and is just good etiquette.
All of the Leagues have an "Intro and Info" topic that will get you started with the League. I'm not familiar with all of them - some are open, some are exclusive or by invitation - but the American Touring Car Championship will be gearing up in November, which gives some time to run some club races ahead of signups.