To activate your card, scan the QR code on the mailer attached to your card. Then, simply follow the instructions in the app.
If you no longer have the mailer, simply log into the app, go to the Activate Your Card section of the Home page and tap Activate. From there, follow the steps to complete your card activation.
If you can't find the Activate Your Card section on the Home page, you can also navigate to Settings -> Card Settings and then change your PIN