Which layout the lessons use
Our Arabic lessons and the on-screen keyboard follow Arabic (101), the standard Windows layout in Saudi Arabia, the Gulf, Iraq, Egypt and most other Arabic-speaking countries. It has no dead keys: every key types its letter straight away. The hamza forms أ, إ and آ, the Arabic comma ، and the Arabic question mark ؟ sit on the Shift layer. The lam-alef key (the B key) types the two letters ل and ا in one press, and the lesson counts both. The number keys type the digits 0 to 9.
Windows
Open Settings, then Time and language, then Language and region. Add Arabic as a language, or open the language options of the Arabic you already have, and check that the Arabic (101) keyboard is in its list. In Morocco, Algeria and Tunisia, Windows often adds Arabic (102) AZERTY instead: add Arabic (101) next to it while you practise. Switch between installed layouts at any time with Windows + Space or Alt + Shift.
macOS
Open System Settings, then Keyboard, then Input Sources. Choose the add button, pick Arabic, and select Arabic or Arabic PC. Arabic matches the letters printed on Apple's Arabic keyboards; Arabic PC puts the letters on the same keys as Windows Arabic (101). The lessons accept both. On either Mac layout the number keys type Arabic-Indic digits (١ ٢ ٣); the lessons accept them as well as 1 2 3. Switch from the input menu in the menu bar, with Control + Space, or with the Globe key.
Linux
Open your desktop's keyboard settings (on GNOME: Settings, then Keyboard, then Input Sources), add Arabic, and switch with Super + Space. The Linux Arabic layout puts every letter on the same key as Windows Arabic (101).
Chromebook
Open Settings, then Advanced, then Languages and inputs, then Inputs. Add Arabic and switch with Ctrl + Shift + Space.
Keys without Arabic letters?
A keyboard that shows only Latin letters can still type Arabic: add the Arabic layout as described above and follow the on-screen keyboard, which shows where every Arabic letter sits and highlights the next key. While your device is set to another language, the keys type other letters, so switch to Arabic before you start a lesson. See what the on-screen keyboard is for. If keys stop highlighting, read how to fix the keyboard not highlighting.