You can activate your physical Revolut card as soon as it arrives through the mail by going to the 'Cards' tab of the app and entering your 16 digit card number.
For your own safety, contactless and online payments of your physical card are only enabled after you activate the card. You can also manually enable them via the 'Security settings' within the 'Cards' section of the app.
If you've ordered a virtual card, it does not need to be activated, and it can be used right away for online transactions.
You must verify your identity before you can activate your card.