Ramp makes it super simple to activate your physical card once it arrives in the mail. Before you activate your card, you will need to complete your Ramp account onboarding (make sure you've accepted your invite and followed the subsequent steps to activate your account). There are two ways that you can activate your card:
From Ramp.com - After you receive your physical card in the mail, log in to your Ramp account and click on the card icon in your dashboard to activate it. You'll need to enter the last 4 digits of your new card. Once you do so successfully, the card will be activated. 
Via SMS - If you have already added your phone number to your Ramp account, you can text the last 4 digits of the card number to 447267 to activate your card with the number registered to your Ramp account. (If you aren't able to activate your card via SMS, it may be because you haven't added a phone number to your Ramp account!)
If your card has been activated successfully, the card will show up as Active in your dashboard and you will receive a confirmation text, which means you can start spending on your Ramp card.