If you're starting your journey in Triathlon, you're probably going to need to set up a new account so go ahead and click the "Create an account" button.
If you have a British Triathlon account, maybe because you've signed up to GO TRI already, you've been a British Triathlon member or event organiser, or because you've purchased an event day membership through British Triathlon, you can just log in with your British Triathlon Account credentials. Reset your password if you're having issues.