"I eat a fish sandwich."
Translation:Ich esse ein Fischsandwich.
Fisch is masculine, and you would use "einen" because Sandwich is masculine as well, and you describe the last word in compunds, not the first.
Both "das Sandwich" and "der Sandwich" are fine. Personally, I use "das Sandwich" and I believe it's the common form. Subway in Germany uses "das Sandwich" as well.
Okay, that I get, but it corrected me by saying that Fisch is neuter, which is just incorrect.