Level as many classes as you would like. You'll gain certifications from each class and combine these certs to make your character stronger. Join a guild - preferably an active and friendly one. Doing so will hopefully allow you to experience the game better in the long run, especially Territory Wars.
Having multiple classes will also add what you can do in Dungeons. But for now focus your main as you are new and don't rush! I rushed as a low leveled and had to go back to 3 low level dungeons to do lvl 50-55 trials. EE is a good game you'll enjoy, make a lot of friends, and don't forget to have fun