When you are new there are several rules to read in the championship's thread, but you will quickly get into the loop because we are quite constant with them
The individual rules for picking cars are on each sub-forum's dedicated thread. For each race, a new thread is opened in the subforum and the available cars are posted there, for a first-come first serve assignment to interested drivers. On sunday the F1 Zandvoort race thread will appear, so I suggest you read the F1 rules and consider asking for a car there when it appears - but be quick when it does
In any case, you are welcome to pick any car of your choice to lap in the server during the week prior to the official sunday seasons (Qualy/race) as that will allow you to become familiar to the other drivers and show your own pace, which can have an effect on car allocation later.