Typing in Arabic (or other Arabic script languages) on most modern computers uses an Arabic layout keyboard. The exact layouts vary between operating systems, look at the differences here. As with other scripts using a unique keyboard layout, while you are learning to type it may be helpful to buy pre-cut stickers for your keyboard so you can look down while you type. They are inexpensive and widely available online.

There are also options available for Romanized typing, where you can spell out the word in Roman characters and the system will automatically convert them to Arabic script. 

• Microsoft Maren: A new Romanization system for Windows to type without learning the Arabic keyboard layout
• Yamli: An online Romanization typing tool