To reset your password, you just need to click on 'forgotten password' when logging in. Enter the email address you use for your Zalando account and click send. You'll receive an email with a password reset link in it (please note: this email may take a couple of hours to reach you and could appear in your spam / junk folder).
Click on the link in the email and you will be prompted to enter a new password.
You can then use your new password to login to your Zalando account.