やる means “to do some action”. You might know that both やる and する mean “to do” but when you refer to a particular action without stating exactly which one, usually やる is used.
んだ is the broken version of のだ. When it’s used in a question sentence, it implies the speaker wants clarification based on the background or context. So in this scene, the context is that Bertolt sees Reiner talking about a secret they never told anyone and he wants to clarify that they are really going to reveal the secret as Reiner didn’t tell him to, directly or clearly.
な is the sentence ending particle which indicates request for confirmation or agreement from the listener in male speech. In female speech, ね is used instead.
今 means “now”.
ここ means “here” and で is the particle for a location where the action will be taken.
So the whole sentence is supposed to be 今ここでやるんだな？ meaning “Are we doing it right now right here?” but he separated the words and changed the word order to make sure that they are really going to do it “RIGHT NOW” “RIGHT HERE”. The nuance gets stronger as you add the words. It also sounds more dramatic than the normal word order.