If you want to level quickly it is best to work through each of the Achievements on the Home page. Try not to spend money early in the game making your zoo look the way you want it to as you will soon run out of cash. If you stick to doing the Achievements in order you'll maintain a steady cash flow. If you keep your Zoo's grade at A you will notice that the top of the Shop page will suggest that you purchase a particular item (e.g., a kiosk). Buy these recommended items before buying animals for your zoo because animals are pricier.