I play Jarvan as Tank. First i start with Doran's shield because in the beggening you have health,armor and health regenerate that helps in getting first blood. Then i buy boots for speed to catch up to the enemy and run for my life. Then after that i buy Last whisper because it has nice damage plus armor penetration that helps.. After that i buy Randuin's Omen for extra health and armor also health regeneration. Right after that i grab Banshee veil for magic resistance and health. Then from all the health i have i buy Atma's Impaler to convert all my health to damage. Then i warp it up with a Frozen mallet helping in attack damage and health and that health adds up in the Atma's Impaler.

Abilities

Dragon Strike - Jarvan IV extends his lance, dealing physical damage and lowering the Armor of all enemies in its path. Additionally, this will pull Jarvan to his Demacian Standard, knocking up enemies in his path.
Jarvan IV extends his lance, dealing 70/120/170/220/270 (+) physical damage and lowering the Armor of all enemies in its path by 10/14/18/22/26% for 3 seconds.
If it contacts his standard it will pull Jarvan IV to its location, knocking up enemies in his path.

Golden Aegis - Jarvan IV calls upon the ancient kings of Demacia to shield him from harm and slow surrounding enemies.
Jarvan IV forges a 50/100/150/200/250 (+20/25/30/35/40 health for each nearby enemy champion) health shield for 5 seconds, which slows surrounding enemies by 15/20/25/30/35% for 2 seconds.

Demacian Standard - Jarvan IV carries the pride of Demacia, passively granting him bonus attack speed and armor. Activating Demacian Standard allows Jarvan IV to place a Demacian flag, granting these benefits to nearby allies.
Passive: Jarvan IV has 10/14/18/22/26% bonus Attack Speed and 10/14/18/22/26 bonus Armor.

Active: Jarvan IV throws a Demacian flag to a nearby area, dealing 60/110/160/210/260 (+0.8) magic damage to enemies, and granting surrounding allied champions his passive bonus for 8 seconds.

Cataclysm - Jarvan IV heroically leaps into battle with such force that he terraforms the surrounding area to create an arena around them.
Jarvan IV heroically leaps at an enemy champion, dealing 200/350/500 (+) physical damage and creating an arena of impassable terrain around them for 3.5 seconds.

Jarvan's Passive is Martial Cadence - Jarvan IV's first attack on a target deals 10% of their current Health as bonus magic damage (max: 400 damage). This effect cannot occur on the same target for 6 seconds.

Tips (Might come in handy)

1. You can use the Dragon Strike / Demacian Standard combo to get out of your own Cataclysm arenas.

2. Split your basic attacks on different champions at the beginning of fights to maximize damage.

3. Use Demacian Standard as a scouting tool when worried about incoming enemies.

I hope you will like my build and dont forget to leave comments.