When you sign up for a Pokémon Trainer Central account, or change your email address or password, you will be sent a confirmation to the current email address you entered when you created the account.
If you do not see the email, please check your spam folder to make sure that the email was not filtered by your email provider. For Gmail, check the "promotions" tab, as sometimes our mail is identified by the Gmail system as promotional.
Be sure you’re checking the email account of the address you submitted when signing up. If it is not there, you can have the email resent by logging in to the site and selecting Resend the Activation Email Now.