Your certificate can be accessed simply by logging into your My Red Cross Account and selecting "My Profile" > "My Certifications". Here you will see all certificates gained under your account.
Additionally the Red Cross will have sent you an email (Subject Line: Your new Red Cross Certification) with steps to log in or register for a new account. In this email you are also supplied with a PDF version of your certificate for instant access.
If you are registering for a Recertification class, it's a good habit to review your certificate expiration date before registration.