Super Mario Run: How to download Nintendo's new game
Android users are out of luck, for now

Super Mario Run is going to drop any time now. (Update: It has now been released) And it will bring with it a new era in the iPhone and Nintendo: the first time that the two of them have worked together.
The game will be released through the iPhone App Store. It's supposed to arrive on 15 December, and always has – but the company hasn't said one time it will be available in the App Store.
Thankfully, Apple has done something unprecedented and apparently specifically made for Super Mario Run: added a "Notify" button to the game so that you can get alerted as soon as it hits. That's done simply by searching for Super Mario Run in the App Store, and clicking the orange "NOTIFY" button that stands where the "GET" option would usually be.
It's not thought that the button will trigger the download when it's released, so it's still important to keep an eye on your phone and get downloading once it arrives.
(In the meantime, the App Store is offering a Super Mario Run sticker pack, which you can download to help keep you going until the app hits.)
When the game does get launched, it's likely that it could have some problems. The game relies players to stay online and so the huge expected demand might trigger some issues early on.
Android users are out of luck, for the time being. The game is expected to be released for the platform at some point, but for now is exclusive to iOS.
It might be possible to play the game through an iOS emulator for Android. But it's not likely to work well, involves downloading the game illegally, and it's probably going to be better for it to wait for the Android release date, which hasn't yet been publicly announced.