If you haven’t been working as of until now, you won’t have a Forskudsopgørelse, and due to this, you won’t have a tax card either. In this case, you’ll have to pay 55% in tax since SKAT won’t know your right tax percentage.
You’ll receive a tax card by creating a Forskudsopgørelse at SKAT.dk. If you want the tax card to be active immediately, you’ll have to edit the Forskudsopgørelse within the same month (SKAT will automatically set the changes to the Forskudsopgørelse to the next coming month!)
You can edit your Forskudsopgørelse by clicking on this link: https://skat.dk/skat.aspx?oid=2234503&ik_navn=subtree
If you were looking for a guide on how to request a new tax card (which is different), you should click on this link: http://help.salary.dk/da/articles/1631446-aendring-af-skattekort-pa-medarbejder