Go to the TOEFL website.
You can register online, via telephone or by post. To register online you need to create a user account.
Once you have created a user account, you will be referred to the page where you can search for available test centres (e.g. Aarhus or Copenhagen) and dates. Select the desired test date, and the rest of the procedure should be straightforward. If no test centre and dates are listed in response to your search, this means all the dates are sold out.
The fee is USD 275 (2018).
TOEFL has lots of resources for students on their website. Go to TOEFL preparation website.