Hi Andreas,
The base RaceRoom game is free to download and play, and includes a few free cars and tracks. It used to be that you could install it as a standalone package, but now it can only be done through Steam, so you'll need to have that installed too.
Firstly go to the RaceRoom home page and create an account:
http://game.raceroom.com/. From here you can then click on the Play button which takes you to the Steam store page from where you can download and install the game.
Once you have the game installed, there is a store button from within game which takes you to another store page which then lists all of the extra content available:
http://game.raceroom.com/store/. If all you want is Bilster Berg, you can purchase it here for a few Euros and you're done.
If you decide that you want a few more cars and tracks, you will be better off first buying VRP (virtual race points) here:
https://www.raceroomstore.com/shop_en/list_page?cat=24 as this will give you more for your money. For the best value, buy one or two of the discounted packs shown in the store, and for the best value of all wait for one of the sales.. And never buy anything directly through Steam - this is almost always a poor deal. It is also possible to try any of the paid content cars from within game before buying them.
The pay model is a bit confusing initially, but I hope that helps. Any more questions, just ask away