You will have to visit the track you will intend to race at... most tracks follow a certain protocol, like certain size numbers/letters on the front and back windows... then your dial, and if applicable the class you are racing in...
The local track may also have a 'book' with a list of numbers you can use, but will have to pay a fee for that number for the season... certain Divisions or pro classes retain certain banks of numbers and you will not be able to get a number in that range...some smaller tracks may not keep track of numbers if they don't have an official race as long as there is not another racer that has that number...
Maybe someone who races more locally can tell us the protocol for the tracks nearest you.
You are going to have fun....I can tell you that.
