One important thing if you want to level up is to make sure you complete your daily activities (quest button in lower right). They give a lot of xp which will increase your player level more quickly.
Also, in terms of character strength, gear level is extremely important in this game. For example, a level 40 character with gear four is probably a lot more powerful than a level 45 character with gear level one.
Sorry if you know all this already, I’m just thinking back to what helped me when I was starting out.