So, first off this is my first guide that I've done, so I'm not the greatest with how to do things on this website, for which I am sorry.

Jarvan I find is able to be played two ways: tank, or physical dps.
Now, most people I've seen so far have played him dps, in fact I tried that my first game with him, however I found that tank works much better for him (in my opinion). Think about it, his q/e move combo is what you'll use to initiate team fights, it gives u that starting edge of a stun (well knock-up), some damage, and your flag's bonus. Next your going to be in the midst of your enemies for the first few seconds before your team joins you, and if you're squishy, you want to be able to survive. For his ultimate...that could definitely be seen as a dps-oriented move - it's a 1v1 with someone right? Well, yes it is, unless you're using that to initiate and you trap, say 3 people, and then your team, possibly ranged and magic, helps doll out some massive damage, and you don't want to die right away (I'm assuming?).

Pros:
Can deal some nice damage
Survivability
Support
Initiation

Cons:
Targeted (I'm not sure why, being a tank, but every game I've played I've been targeted right away - probably because it's a new char, in which case this won't apply soon)
Very, very, slow - hard to get away
No aoe, or high damage spells

Changed build so that I get Frozen Mallet after sunfire and warmogs. Find that I like that better survivability first from the sunfire cape (armor and health), and it is nice with farming. Warmogs can be gotten after or before mallet, it depends on what you want to get. I like it before because it just is better for survivability.

For summoner spells:
I choose to have exhaust and teleport.

Why exhaust?
First off, you're slow, probably your fall for wearing all those dragon bones. Regardless, exhaust will let you get away from people who are faster than you (so really most people), as well as catch up to them. It is also good in a 1v1, so it works great with your ult.

Why teleport?
It's just a nice spell to have. I find that I'll often get the first kill or two, with my partner, before the other team kills us, and that'll leave me sorely hurt. It's nice to have the option to go back and heal, purchase, and lane right away (remember he's slow!). Also, if you need to defend some tower ASAP, it's great to have.

Other options:
Haste - you're slow (and I'm going to keep saying that), haste is nice for the same exact reason as exhaust
Flash - always a nice spell in a pinch, will give you that extra jump you need to get off ult, or to get away
Ignite - You can if you want, but it's not my bowl of soup. I find that it's an annoying spell when it's used against me, and I've even gotten some nice kills with it in my day, but it's not actually as helpful as the previous mentioned spells.

There's really no other spell you should get. If you want fortify, or heal, you can get it, but frankly, in the long run, I think that it's not really as useful.

I'll be perfectly candid, I've never saved up enough points to really max out rune pages, I've been too busy buying champs. However, this is a solid build, in my opinion, as far as the runes go. This dodge is amazingly helpful, even though it's very small amounts of it. There are almost no items left with dodge, and even with masteries, you get very little. This extra boost is a big help, especially if you have 2 or 3 guys tearing you down. Armor and MR, early game this is going to be wonderful. Trust me, this'll give you an edge, making it a LOT harder to hurt you when people don't have much damage output. Finally, since I do go damage a bit, this armor pen is nice to have, though you can do whatever you want really.

Masteries!
I go 0, 22, 8

Utilities:
I get teleport, it's why I pick that one, though if you're not getting teleport, you probably don't want to get it...I get the bonus exp, cause it's really nice, and the magic/healing is just a lifesaver early game. As for the defense...I think it's really pretty self explanatory, everything a tank wants, those 28 got it.

ITEMS!

SO I start off with boots and 3 health pots. Why? BECAUSE I'M SLOW! I'm generally against doran items because I don't like to spend that much money, when it could go towards building my first item. The health pots should keep you in, with your masteries, really as long as your going to need to, especially if you have teleport (then you really don't need to worry about life regen imo).

Boots:
Entirely depends on situation. If they have a lot of CC, get mercury, if they don't, get swiftness (you're slow!). If you're going mid, get mobility.

Frozen Mallet:
This is great for him I think. Gives you slow, so they can't as easily outrun you. Gives you health which is nice already on a tank, but mixed with atmas later is even better.

Sunfire Cape/Warmogs:
I'll get either this next or warmog's. I'll buy the belt and see. Do I need armor? If I don't I'll get Warmogs cause that health and regen is lovely on any tank (and boy does it give you health). Sunfire Cape is nice EVEN IF THEY DONT HAVE MUCH PHYSICAL DAMAGE! Think about it. You're going to e/q into them, or you'll ult, no matter what you're going to be in the middle of a bunch of them, this will deal out damage to everyone around you, and will give you health AND will give you armor, not bad eh?

Thornmail/ForceOfNature/Randuins:
Entirely situational.
Do you need major armor, and are they all melee? I'd say thormail's the item for you!
Do you need major magic res, because so far you have almost none and they have 4 mages! No item is better than this (and the regen is great :D). Or perhaps you need some of everything, if they're a very balanced team, I say go with Randuins, even though it doesn't give you the MR you might need.

Atma's:
By now you'll have something like 4500hp or so, maybe a bit less, depends on what items you got exactly, how long you've had warmogs, etc. You are a damage champ too, it's why I think the most popular build is dps, and so I bring in atmas. This is super nice defense wise if they have melee, and if they don't, hell maybe you want to change a different item for MR (even sunfire), but this is a must! what's 2% of 4000? 80. But you'll probably have more than that, so it'll be even MORE damage. 80 damage (plus some crit) from one item is pretty nice, and when you add that to the frozen mallet, you'll be doing around 200+ damage.

Last, but not least, Skills!

I like to start with flag. Why? Because it gives you a really nice passive boost, and it can do some damage Next I go with Dragon strike for the wonderful e/q combo. I proceed to max out flag asap, switching off with shield, and of course your ultimate. I max dragon strike last, because the damage output is small, all things considered, and since I get atmas last, the majority of my damage comes late game.

There you have it, Jarvan! so far...
Hopefully this build will come in help, I've never done one before, so perhaps it sucks...I guess I'll see. I'll try to update it if I figure out how, which frankly won't be difficult, if I find a better guide, seeing as how Jarvan's been out for about 2 days, less?

Thanks for reading, if you've made it down here.