Vordt Of The Boreal Valley is the second boss you will encounter in dark souls 3 and he can prove quite a challenge if you don't play smart.
Vordt can break your shield guard in one hit so don't bother using your shield. Vordt also hits very hard and he can cause frost damage which is a new element to dark souls 3 which will slow you & make your stamina bar regen very slowly. To beat him, try get behind him and stay there if you can as he wont be able to hit you here, locking on and rolling left is the best way to do this.
Midway through the fight he will Agro and start charging at you multiple times. The way to tell when he is going to do this is by his eyes glowing red so simply dodge then continue your attack. Attack, Evade, Repeat.
Below is a video guide: