I need help, my fellow Hetalians! ;u;;. Please answer this <3.

((Awwwh. Thats adorable. cx

Well, you have to be rather obnoxious and prideful. Have sort of a "me" obession. (There's a reason why Denmark, America and Prussia are the awesome trio) That is his personallity basically.

You should look up clips of him (there arent that many ;3;) or maybe art abd stuff. But I mean if you could pull off the accent thats impressive, ad pretty cool. But hey, personallity is first!

Good luck to you! I'd love to see photos of your cosplay!))

Well since @Natalya_Belarus talked about the acting and personality i guess I'll give you tips on appearance :P
So, stalk up on gel cuz you need that spiky hairs of his. Also, Denmark usually wears dress shirts but they are usually not fully buttoned as that would look to formal!! Hope that helped ^-^

The pic is so kawaii <3

Well, I would recommend really getting to know as much of your character as you can. Research Denmark's personality and such. Denmark's personality is playful and overconfident. He tells tall tales, as seen in Episode 2 of The Beautiful World, where he talks about the Nordic Robot.
As for the actual cosplay itself, I'd say you'd better stock up on hair gel. xD
And one last tip: Make sure you feel comfortable in your cosplay! Make sure you're happy and content with the final result. :)
Good luck~!