There are five different character classes. You start off as a greenhorn. As soon as you reach level 10, you can choose one of the other four classes. But be careful, the selection is permanent and cannot be changed later. The selection of the character picture and gender is also permanent.
Choosing a class
To choose a character class, click on the blue circle in the character menu. You can have a look around and carefully consider your choice. You can also select a character picture and a gender. The picture and gender don't change the advantages of the classes. Once you've chosen everything, select the character picture and click "Choose character class". As soon as you've made your selection all advantages become active and the picture is changed.
You can stay a greenhorn as long as you choose to. It is not recommended though because you would miss out on all the advantages.