i'm new at trade so...

What did you buy the deus ex hr key with? You can possible charge it back and get his account banned

Guys please where i can report him

Just so I'm clear on the details, you sent him a tradable copy of Deus Ex: HR Augmented, expecting him to send you back an Arma key that he never did?

First and foremost, DO NOT CLOSE THE CHAT WINDOW. Screen cap it, copy the chat log, save both. There is almost no chance you'll get your game back, but you can prevent the scammer from doing the same to other people. Take the evidence of the trade and report him on Steamrep and to Steam support and there's a decent chance he'll get marked as a scammer and trade banned.

For reference, here is the static ID information for the account you linked in case they change their custom URL.

Gazuotasz (76561198058014479)
Gazuotas (76561198060299213)
Mantvydas (76561198060794064)
Mantvydas014 (76561198061234076)
Nko (76561198063689612)
Regal (76561198064614320)
Buypcgame (76561197976330988)
May (76561198067044928)

Let's just say, he has a lot of alternate accounts and just got banned hard.