There are the famous few fast food restaurants e.g. Subway, Macs, Burger King, Starbucks, Pizzahut & KFC.
However in Germany, the popular fast option would be a bakery. All bakeries sell various assortments of breads (brötchen), either plain or as a small sandwich (belegte brötchen), various types of pastries, as well as coffee.
Another type of fast food would be kebab. Most kebab places are walk-up window stands where you place your order with the toppings you would like.
There are your typical fish & chips stalls around, known as Nordsee. They offer fish plates with pommes frites (french fries), and possibly some vegetables.
Lastly, there is the popular Currywurst. It is a pork sausage (bratwurst) that is cooked, sliced and covered in curry sauce, which is a type of ketchup with curry powder mixed in.
There are regional food around as well, e.g. pretzels in Bavaria so keep an eye out for them 🙂