I have found that going for a life steal/attack speed build with Jarvin works fairly decent.

Those Runes aren't set in stone. Tweak them how you like but stick with Armor Penetration, Health and Magic Resist.

For the Summoner Spells I generally go for Clarity and Ghost.

Clarity is a must so that you can lane for longer.

Ghost for chasing and running away.

Exhaust, Flash, or Ignite are viable alternatives. Depends on your play style.

Level up your Demacian Standard ASAP, then Dragon Strike, then Golden Aegis. And of course level your ult whenever you can (At lvl 6,11,16)

Starting out with Dorans shield is makes it where you can lane longer, and be more gank resilient.

Basic boots is enough early game.

Then you start going for your Hextech Gunblade. The life steal side of the Gunblade (Bilgewater Cutlass) is the most important.

After you done with the Hextech Gunblade, you will most of the time go for Berserkers Greaves. In games where you are getting beat up go for Mercury's Treads.

Now you start on your Phantom Dancer. I generally get a Zeal, then get Thornmail, and come around and finish off my Phantom Dancer. If the enemy team is going more AP, then go Guardian Angel. Sometimes if I get my armor before going for my Zeal, depends on how much damgage the other team is doing.

If the game lasts long enough, get rid of the Dorans Shield and go for a Black Cleaver.

If the in the game you are starved for Mana, go for a Shurelya's Reverie.

HAVE FUN! THATS WHAT GAMES ARE ABOUT.