When you make a new account on PokeBase, you will be sent a message titled "PokéBase Pokémon Answers - Email Address Confirmation" to the email address you gave when you signed up. Click the link inside that message to confirm your email.
If you gave the wrong email address (e.g. made a typo), you can go to your account settings to fix it.
If your account isn't new or you recently changed your email (or are having trouble for any other reason), you can use this link to send another email.