Pokémon GO is available on the iPhone and Android. From the home screen of your phone, swipe your finger to the right and type "App Store" (or "Play Store", on an Android) into the search bar that appears. Tap the App Store icon to open the App Store. Tap on the Search button at the bottom of the screen and type "Pokémon GO" into the search bar. Tap Search to see a list of results pop up. Find the Pokémon GO app in the results. Tap the GET button at the top-right corner of the results bar. You may be prompted to enter your Apple ID password. After doing this, you should see the app begin to download. Press the home screen button and tap on the new Pokémon GO icon that will have appeared. If you don't see the app on your home screen, swipe left until you see the Spotlight search bar appear where you can type "Pokémon GO" in and tap on the app that appears. Giving the app access to your location will allow you to make full use of the game's features. When you have done this, hit Submit. You can do this one of two ways:   Sign up using Gmail. If you have a Gmail account, you can choose this option to link your account to the game, allowing you to share data between the two accounts. Signing up with Gmail currently seems to be more stable than using the Pokémon Trainer Club.  Sign up for the Pokémon Trainer Club. This is a feature on Pokémon.com that seeks to create a dedicated community of Pokémon players who can communicate, battle, and trade Pokémon with one another. If you're interested in joining this community, this is a good option to choose. After accepting the terms and conditions and an introduction from Professor Willow, you'll be taken to the picture of two avatars.  Tap your preferred presentation and you'll be taken to a screen where you can edit different physical features of your avatar. Edit your features by tapping on the different sections of each and using the arrows to toggle between different looks. Tap on the checkmark at the bottom-right corner of the screen when you are finished designing...

TL;DR in Vietnamese:
Vào kho ứng dụng App Store. Tìm ứng dụng Pokémon GO. Tải ứng dụng Pokémon GO. Mở ứng dụng Pokémon GO. Cho phép Pokémon GO truy cập vị trí của bạn. Nhập ngày sinh của bạn. Đăng ký tài khoản trên Pokémon GO. Tùy chỉnh nhân vật huấn luyện của bạn.