This Jarvan build is fairly basic and along the lines of how Jarvan was originally intended to be played. However, it's simplicity is also it's strength - making the build easy to learn. I love playing Jarvan because despite his off-tank nature, being able to take quite a few hits, he can also deal out a fair amount of damage. Using these two facts to your advantage can be the difference between winning and losing the game.

The masteries i usually go for are a basic 21/8/1 build. I use points towards damage and attack speed to strengthen his core abilities while also spec-ing into armor and dodge on the defense tree. My one point in utilities is optional as it is just to buff my Ghost summoner spell.

My item formation is my favorite part. The sequence goes like this:

1) Dorans Shield
2) Boots
3) Frozen Mallet
4) Warmogs
5) Atmas Impaler
6) Warmogs

1) The Dorans shield, although a very basic item, provides that extra armor and health at the beginning that will put you above most mages and melee opponents.
2) I usually just start with normal boots then head straight to the Frozen Mallet, it takes a while to figure out whether you'll need more armor or attack speed judging by their teams dynamics.
3) The Frozen Mallet gives you that extra health and damage that you'll really start to be thankful for come mid-game. You should get this before lanes are over.
4) The Warmogs Armor serves a dual purpose. Not only does it support the off-tank nature of Jarvan IV but it also sets up his Atmas to increase the effect.
5) You should have Atmas by the time you reach end game or at least by 30 minutes. The Warmogs Armor that you just bought will be paying off now. You should have roughly 3000 health and 2% of that will head towards your Attack Damage.
6) Moving on, a second Warmogs should not be necessary at this point but if it turns out to be one of those games that just seems to be never-ending, more health will once more serve a dual purpose.

The skill sequence is fairly simple.

Use your E skill (Demacian Standard) to increase attack speed, etc.
When combined with your Q (Lance), you will be rushed to your Standard, knocking up all opponents in your wake. This will also decrease their armor by a fair amount.
If you're very confident of your abilities and your chances against the opponent, you can block their escape by using your ultimate (Cataclysm) which will make his/her death fairly quick and painless. When using Cataclysm, do beware of both ranged opponents outside of the circle, other opponents with grab skills and opponents with pounces. Choose your target wisely. I usually find that the ranged mages or the parties main focus are your best bets - feel free to coordinate with your team.

Okay. Summoner Spells.

For my play style this is fairly simple, I utilize both Clarity and Ghost to their best capabilities:

Clarity - Having clarity allows you and your laning partner (this is a team game) to remain in the lane for longer whilst being able to put fairly constant pressure upon your enemies (if your partner has decent damage capabilities). Come mid - late game it can come in extremely useful to support allies in tight spots or give you that little kick in team fights to win. We all hate it when our main ranged or melee damage outputs run dry on mana.

Ghost - Ghost is fairly self-explanatory. It's a useful summoner spell when you need a quick escape speed boost or just to maneuver back to a lane or gank more efficiently. It's an all around handy tool.

Although I very much prefer said skills, mess around with your own play styles and find a way to balance your talents alongside your champion!