وعليكم السلام
You don't need to buy anything, it is available as part of the iPhone.
<ol class="_jYe" style="margin-right: 0px; margin-bottom: 0px; margin-left: 4px; padding: 0px; color: rgba(0, 0, 0, 0.870588); font-family: Roboto-Regular, HelveticaNeue, Arial, sans-serif; font-size: 14px;"><li class="_AXc" style="margin: 0px 0px 12px 16px; padding: 0px; list-style: inherit;">Go to settings.</li><li class="_AXc" style="margin: 0px 0px 12px 16px; padding: 0px; list-style: inherit;">Go to “General”, then tap “Language & Region”</li><li class="_AXc" style="margin: 0px 0px 12px 16px; padding: 0px; list-style: inherit;">Tap “Keyboards” > “Add new Keyboard”. At this point select the language you want to add from the pop-up window. The keyboard will then be added. You can add more than one extra language.</li><li class="_AXc" style="margin: 0px 0px 12px 16px; padding: 0px; list-style: inherit;">You will now see a little globe icon on your keyboard. Press this to change between languages.</li></ol>For the computer, it depends on which operating system you are using but again it should be free.
To learn to type you can open Google translate, set it for Arabic to English, and you'll see, at the bottom of the little window, the symbol of an ع . If you click on it you will see the symbol of a keyboard. Open it, and type on your keyboard (Not on the virtual keyboard, but on YOUR actual keyboard), while looking at the screen at the virtual keyboard and typing into the Google translate window. Like this you'll learn to type without looking. I can now type in Arabic without looking. It's great.
Google translate also gives you an instant translation and therefore it helps you notice any errors more easily.