"합니다" directly translates to "Do". So if you say "저녁을 합니다" it would mean "Doing dinner".
It might sound acceptable in some cases for English, but in Korean that would sound quite awkward. (Colloquially)
Instead we say "저녁을 만듭니다" = "Making Dinner"
So if i were to say "I'm going to make dinner at six" = "여섯시에 저녁을 만들겁니다"
"겁니다" is future tense.
I think this is common to many of the questions that people ask about in Korean grammar. The word at the end of the sentence normally determines the tense of the whole sentence.