Open Settings > General > Keyboard > Keyboards > Add New Keyboard

Select Japanese from the menu and then select the kana keyboard and/or the romaji keyboard. 

Afterwards, when you open your keyboard, you will see a small globe symbol to the left of your space button and mic button. You can tap this button to toggle between languages. If you selected both the kana keyboard and the romaji keyboard, you can switch between the flick kana keyboard and the QWERTY keyboard by long-pressing the globe button.