Hi :) Welcome to the game, to gain exp is pretty easy :) You gain exp by beating others in attacks and by doing quest,
To level up you need to fill your exp bar twice to gain one level.
You can choose to train strength or defense (exp bars (both count as exp) by attacking players or boss's (boss's are more important) and quest only train exp :) there is a exp bar on the top of the page to let you know your exp (and on your profile)
for more info just ask the clan you choose to join or pm me in the game
Hope this helps and hope you enjoy the game :)