You can buy diamonds right in the game! Otherwise, you get a diamond when you finish reading a chapter. You can also keep an eye on Daily Challenges (ribbon icon located at the lower-right corner of your screen). Other than that, you can tap on your Diamonds counter in the upper right corner of your screen and after you do so, you should be offered to tap on 'Get FREE Diamonds!'. From there, you can watch ads or complete some offers (if they're available) in order to get Diamonds.