This is my first guide. I hope this guide helps you in playing a proper Jarvan IV cuz Demacia needs you. This guide is meant for top lane.

I will do my best to lead you in the path of Demacia.

Pros:
Strong harasser early levels
Natural Tank (from his E)
Decent Farmer
High Mobility
Good crowd control
Can turn team fights
Cons:
W wastes mana but sustains in lane
One of the hardest champ to master
Needs farm and kills
Often judged by Adc's to not farm and play support =P

I go flat Ad runes when your top lane. You Q lowers armor but also increases damage from AD and his ult. Pretty good reason.

I'm pretty sure you want to play pretty tanky, but not like super tanky so u can't kill anything (happened to me before). I go 9/21/0 for masteries, taking more defence then offence. I see Jarvan being more defensive so he can charge in and harass without being squishy. Have defence masteries lets you survive better against odds.

When farming always start with boots and 3 health potions which get useful. Always try to last hit without using your skills. Against enemy champions use Q to harass and lower armor and then autoattack dealing massive damage. I consider taking E first then Q to check the brush. Save your Q and E combo for escaping or knowing the perfect time to gank. Using E+Q to harass wastes mana drastically, lower your options and pick your chances wisely. Later on Jarvan doesn't really need to rely more on mana unless in teamfights and escaping.

Offense

Trinity force
Black Cleaver
Atma or Maw

Defense:Defense:

Warmog (needs sustain)
Guardian Angel (if tooken down first)
Atma or Maw (also offense and defense)

And Obviously Boots I'd say Alacrity Mercury Boots