If you want to develop for Android, you can download the Android SDK from:
https://developer.android.com/studio This will allow you to build Android apps that you can then test on an emulator or on a real device.
To build apps for iOS, you'll need to download Xcode on a compatible Mac:
https://developer.apple.com/iphone/ If I'm not mistaken, I think that Apple will let you test apps for free on real hardware. However, if you want to put them on the App Store, you have to pay $100/year for a regular license, or more for a business type of license.