Jarvan IV is capable of pulling off many different builds. From jungling to tanking to DPS, Jarvan IV is a great champion to use.

I'm not going to teach you how to play as him, simply recommending you use these items in a game and just watch your stats skyrocket.

First thing's first, this is a DPS OFF-TANK build. Don't try and jungle with it.
And incase you were wondering, an off-tank is a half tank half AD champion.
Before you use this I recommend:
-Do NOT spam out your Demacian Standard(Flag) early game, it will drain all of your much needed mana.
-Follow this build according to who you're playing against(e.g if you're fighting against 2 AP champs, don't bother going for Frozen Heart after the Ninja Tabi. Go for Banshee's Veil instead. Don't be scared to switch the build around.)
-Late game, make sure you buy several elixirs of Agility to improve you attack speed, it really helps out when you ulti, or when you're trying to take down a turret.
-Also, I sometimes use heal instead of exhaust. it could really change the tide of a 1v1 or a team fight.

I wish you all best of luck, I hope this build helps you out as much as it does me.