You need unity, the android SDK, and a compatible device for testing (optional, recommended). Now it’s not going to work on older devices, support is for most devices in use today though, with exceptions for older android versions that won’t work with the latest unity android publishing process.
Good luck!
Edit: oh also when your done creating your app, you must pay Google 25 bucks USD to publish to play store. There are other platforms as well like amazon.
Edit 2: oh another thing, some older os versions won’t work, but being rooted, and updated to a newer android version (perhaps through cyanogen mod) will maybe allow them to work.