Android Pay is getting support from a number of banks and retailers that seems to grow by the day. If you happen to frequent some spots that take Android Pay and use a bank, prepaid card issuer or credit card that's supported, now may be a good time to set up Android Pay and give it a go. A quick caveat before delving into the tutorial, though - at the moment, custom ROMs don't work with Android Pay, and even the stock firmware that came with your phone must not be rooted. Xperia owners running the Android N preview are likewise out of luck, and Nexus owners on Android N must be on Developer Preview 3, the latest version. Even unrooted pure AOSP builds are out in the cold, for security's sake.
The app will greet you with some intro cards showing what it can do. From there, you'll see a blank screen where you can add a card. Use the plus sign in the bottom right to do so. You can add in any card you've used with Google and saved to your Google profile before, such as letting Chrome save your card during an online transaction. If you have no prior cards, you can take a picture of a card, which will ask you for the 3-digit CVC code to confirm, or you can type in the card details manually. The card will be verified through your bank, then you will be prompted to complete a verification via code, by SMS or email. After that, the card is all set. Keep in mind, however, that Android Pay requires your phone to have some sort of secure lock screen, whether by fingerprint, pattern or otherwise. If you don't have that set up when you add a card for the first time, it will prompt you to put it in place. If you ever change your lock screen, you'll have to re-add your cards.
Once your cards are all set up, including loyalty and gift cards from stores, you can check out the settings for the app, check transactions and the "virtual account number" for your card, which is passed to merchants instead of your actual card number, making Android Pay a bit more secure than using the physical card. In any case, when you're ready to pay in a store, simply unlock your phone, ensure NFC is turned on, and hold the phone against the terminal until you see a check mark. On some phones where multiple payment solutions may be present, such as Samsung devices, you can choose which one to use by default in your settings menu. If you have multiple credit or debit cards set up, the card in the top spot is the default and will be used for payment. If you'd like to use a different card, open the app and drag it into that spot before making your payment.