Fill the upper field with your username (this is your debtor number, it usually looks like this: DBxxxx. the xxxx are four numbers.)
Fill the lower field with your e-mail.
Press the blue button "Verzenden".
Go to your e-mail and open the mail from Tuxis. In the e-mail you will find a temporary password.
Log in with your username and temporary password.
Now you will be prompted to make a new password. Come up with a strong and long password, containing uppercase and lowercase letters, digits and punctuation marks.
You are now logged in and you can login with your new password the next time.