Well the easiest way is to buy some credits or if you really don't wan to for every 5 levels you get a Leader card you can sell them to Kate for easy 800 clintz which can help you get some better cards.
Also participating in daily tournaments helps cause you can get at least 50 clintz and a credit for just joining. Once you have a decent deck play in ELO or Survivor as well.
And please read the game rules and visit the strategy threads to learn how to play this game as well. And I also have to borrow a line from Captain Clintz "the search function is your friend "