This is my first guide ever so please give any feedback and comments you have. Anyway here it goes.

Jarvan is a tanky/dps that means two thing
a) he can tank
b) he can dps
So you can have both!!! (no @#%$@ Sherlock) and since everybody fancies a kill dps a little more.He is tankY and not a TANK! Dont bother with magic resist or armor you are wasting his potential.

If you are not into reading just check the TIPS chapter for a rough guide about Jarvan

Early Game
Level your Demancia Standard in order to scout the bushes and get that kill if things turn out good in your lane.
Get a [Ruby Crystal] just to have enough health to stick to your lane untill you have that 500 gold to upgrade it into a [Heart of Gold]. I recommend to go back and get it as soon as get 500 gold because in the long run you are going to have some serious cash out of it (5g/10sec 30g/min). Also heart of gold boosts your lane survivability.

Once you gather 1350g go back get [Tear of the Goddess] and [Boots of Speed]. Remember to spam your three first skills whenever you go back in order to feed your "tear".

Grab a [Zeal] after that, the extra movement spead is a god sent gift + the extra attack speed

Mid game
At this point you are going to need some extra health since you already happen to be the fight initializer or soon you are going to be. Grab a [Warmog's Armor] + your health runes you are going to have a nice hp pool to walk around and kill staff.A [Leviathans] would also be nice. If you are good enought and score soem assists or kills your next item got to be an [Atma's Impaler].

Late Game
Here basically you boost your physical damage even more. some people would like to go for a [Madred's Bloodrazor] for the attack speed and the stacking up it does with Jarvan's passive but i prefer [Infinity Edge] since you get attack speed from boots + Demancia Standard+Zeal. The game should be over soon but if there is still time finish your [Phantom Dancer]. If the game is longer than usual a Bloodthirster is there for you, then you are practically immortal.

i wont go much into it. armor penetration for physical damage is the way to go.

For the yellow runes i prefere health/level in order to feed [Atma's Impaler] more and increase survivability.

Attack Speed/level is just my thing. So these runes + Demancia Standard is all you need when it comes to attack speed so you can boost other stats

Armor penetration Quintessences for me because these 3 + the Red ones will give you enough so as not to bother with armor penetration items if you are up against normal champs (no tanks i mean by that)

Again here it's a normal physical damage dealer build with the extra points to utility for a little bit of that tanky bit.

Flash is the best spell out there for almost any champion. Use it to get close and use either your E+Q ability or to R someone. It is also very very helpful to get out of squishy situtations and out of your ulti when things are going bad!

Exhaust is just and addition to your shield slow....they practically wont be able to move if you apply both. you can also use it to slow an emeny down after your ulti ends and still you havent got the kill. You can replace it with ignite for more aggresive play!

i max out Demancia Standard at any given chance. It's a nice range damage and the buffs are really good to sustain dps on a target

After that Dragon Strike comes. It has a great damage output and will help you get some nice kills as you level it up

Last but not least Golden Aegis the only reason i dont level it is because i have some basic knowledge and experience on how to survive so i dont need the damage migitation that badly and i use it for the slow most of the times.

i cant tell you how to play Jarvan this is up to you, but here are some usefull tips

-Master E+Q combo litteraly reach a point that you do it without even realising how difficult it might be.

-Dont just do it to charge in..try to master it also while fleeing and you are moving away from an enemy

-E+Q over thick walls!

-Dont be afraid to flash out your ulti you got plenty of damaging skills and attacks you ulti is just one of them

- Yes you can slow people on the other side of a wall with your shield if they are close enought

-This is a guide not a one way road, weight your opponents and buy things according to the situation

-Switch targets when thats doable...use your passive

-You can interupt Katarina's ulti with E+Q and you can stall someone long enought to get hit by the turret..(you have exhaust too!!)

-DONT FEED this build unfortunately requires some cash and timming....so if you are left behind because you die too much in terms of cash you will probably left behind in damage output while others will pwn you over and over and wont feel any pain from your little spear