There are 3 different modes. Arcade, Career, and Networking mode. You need to play career mode to receive all the vehicles. The further you have gotten in career mode, the more vehicles you have to choose from when playing online. I havent played Arcade mode yet, but I assume you can choose from all the cars. Ofcourse, that could also be set up like online mode where you still need to have unlocked the levels in career mode.