You can set up App Inventor and start building apps in minutes. The Designer and Blocks editor now run completely in the browser (the cloud!), so you can start building apps without downloading any software to your computer.
You do need to perform some setup to test your apps. If you have an Android, you need to install an app on the device, the MIT App Inventor Companion. If you do not have an Android device, you need to download some software to your computer so you can test on the emulator. Once you set either of these up, you'll be able to test your apps as you build them!
Test your apps with an Android device and WiFi Connection (preferred) instructions
If you have a computer, an Android device, and a WiFi connection, this is the easiest and coolest way to test your apps.
Test your apps with an Android device and USB Cable instructions
Some firewalls within schools and organizations disallow the type of WiFi connection required. If WiFi doesn’t work for you, try USB.
Don’t have an Android device? Use the Emulator instructions
if you don’t have an Android phone or tablet handy, you can still use App Inventor. Have a class of 30 students? Have them work primarily on emulators and share a few devices.