Complete the Basic and Advanced Tutorial to get 3,500 steel. This will allow you to unlock 7 of the 9 classes at the beginning of the game. You can also buy fewer classes and spend some steel on unlocking "Champion Status" time, which allows to gain bonus XP and other rewards for a limited time. If you are planning on only playing as a single faction, you can simply purchase all three locked classes for the Knights, Samurai, or Vikings.