There are two ways to add a card to your Google Wallet -- from the Wallet app or your bank's app or website. The first option is the most common.
From the main screen on the Wallet app, tap "+ Add to Wallet" at the bottom. Choose the type of card you're adding -- payment, transit, loyalty, etc.
If you're adding a payment card, tap "New credit or debit card." Either scan your card with the camera or type in the information manually. Once you've added the info, tap save. Google may ask you to verify your payment method through email or text.
Also: Google Wallet will now automatically add your movie ticket or boarding pass
If you're using Wallet for identification at TSA precheck, you'll be asked to scan your ID card and then take a brief selfie video.
Additionally, some banks let you add cards to Google Wallet from the bank website or app. If your bank supports this option, you'll see an "Add to Google Wallet" or "Add to GPay" button when you log in. Tap that button and follow the instructions.
If you're adding a transit or a loyalty card, you'll have to choose from the list (not all transit options and stores are available). You can then log in to your transit or store account and connect your card.