You have to download and setup flutter from flutter.dev. You can follow documentation with your own device.
Windows: https://docs.flutter.dev/get-started/install/windows
Mac: https://docs.flutter.dev/get-started/install/macos
Linux: https://docs.flutter.dev/get-started/install/linux
TIP
Recommended setup tutorial is below 👇
Now run a command:
flutter doctor
Then check that every setup is okay. If not then check the documentation again and set up flutter sdk properly.