Here is my build/guide for playing Jarvin. I've seen others and honestly think his role should be played at support and not AD/DPS builds. He is a wonderful support. Very good at initiating team fights, slowing, knocking up, and taking spells from the enemy team letting your champs finish the job easier.



Pros:
[*] Good Support
[*] Beefy and can deal nice damage, especially with passive
[*] AOE Slow
[*] Knock up
[*] Good escape and can close distance fast enough to prevent enemy from esacping
[*] Can trap many champions in his ultimate saving your carries and taking hits for your team

Cons:
[*] Slow Movement speed
[*] Low mana pool, and uses it very quickly.
[*] Ulti can mess up your team if not timed right



Of course as with all builds, it changes with every game depending on the other team. But my core in this is the Mercury's Treads, Randiums Omen, and Banshee's Veil. You want to start out with Doran's Shield, then build boots of speed, then get your Heart of Gold. After that, get Mercury's Tread then your Banshee's Veil, then complete your Randiums. Remember that you are the support and not the tank. Your team should have a tank, if needed then go ahead and grab some more defense if you find yourself dying to fast. If they got a lot of AP then you should grab a Force of Nature, if they got a lot of AD consider grabbing Frozen Heart or a Thornmail.

Another summoner spell that you can use for Jarvin is good old Flash. With this you can flash out of bushes or into fights and use your ulti for fleeing champs.

Things to look out for:
Your ultimate is EASILY counter by the enemy team if they have flash. Make sure you or your mates can harass them enough to cause then to use their Summoner spells. Save your ulti for those who used their flash already. Players like Tryndamere, Shen, Caitlyn, Kat and others can easily get out of your ultimate, so save it for other enemies or only use it if they are low enough that your ulti will kill them.

Farm as much early game as you can. By mid to late game, you should be helping your teammates out and initiate fights. Remember you can use your E to see if there is anyone in a bush before approaching. In a team fight initiate in with E and use your Q to knock them up and immediately pop your W and Randium's active. Remember that your tank should be the one to go in first and you come in after. This will really slow the other team and screw then up as your team comes in and just unleashes on them. Hit the carries as much as you can and use your ulti to either trap the enemy's carry or to save your teammates. He's great for taking out carries and saving your mates.

Remember the speed Quintessences are a must, Jarvin is so slow and since you're support you want to be able to run into team battles fast and catch enemies. A good way of catching a fleeing enemy if you are past level 6 is to use your E then Q in and use R immediately when in range. Remember in fights you want to focus the enemy carries and take most of their hits for your teammates, its ok if you die because if you do die your teammates should have most of their team dead if not fleeing. COMMUNICATION is a must. With Banshee's veil your mana pool should be fine just dont go around spamming your abilities like an idiot and you should always have enough to come back into team fights.

If your not getting shredded right away go ahead and build TF and BC. BC with your q and will devastate your opponents and help your AD carries rip right thru them. To be honest I the other team has never lasted long enough for me to finish Trinity Force, I have never gone that far into game when I play Jarvin. Remember you can easliy kill a fleeing enemy low on health with your E, just aim it right in front of them :)

Overall solid champion, I have played him both DPS and support and honestly, if you got good carries and a good support Jarvin your team will do amazing. He is meant as a support so build him like one. Good engager and can take a lot of hits from the other team. I know this guide is kind of short but i am not too familiar with MObafire format and how to put all that stuff in, but i will extend this guide and explain more when i get a chance and learn how.
Try it out before you comment please, and play smart and i gurantee your team will love you. Sorry its really short and brief but to the point, i will extend it later on