On what conditions your future husband can have a visum for a family reunion to Germany, depends on the kind of your residence permit. The longitude doesn't say everything, it is important to know what paragraph you have in your Aufenthaltstitel.
Anyway you must be normally 18 to marry officially. And if you do it in Denmark, check 1) what the conditions in Denmark are 2) that this marriage will be accepted in Germany. Sometimes people had problems because danisch authorities did it easier, without proper checking the documents, and the Germans didn't want to accept it later.
Your husband will probably apply for a visum in an embassy in Italy, so he will normally have to provide italian documents because he has his adress there. But you must check what the German embassy wants from you. The embassies mostly have Websites where you can find the lists of conditions and necessary documents to provide.
You can also consult a special councelling center for migrants in your area. There are many of them everywhere in Germany.
As a 17-year-old you have a Vormund, perhaps this person can help to find information. And there are also family consultancies like pro familia etc. Each case is individual.