Ok, So I'm tired of people taking perfectly good tanking Characters and making them DPS because they like to see big numbers fly. Jarv is an excellent tank, and still does great damage without buying any real damage items. His passive is a mini Madreds. And will take care of any enemy. And his Q gives enough armor pen to deal plenty of melee damage. Just get him a ton of life so he can take a beating. Now depending on your team comp will determine what kind of items you get. If you get damage items your wasting your main strength which is to go toe-toe with anyone you get in your ult... And you will take damage in it. Jarv is a great off tank, hes best suited to initiate or keep the carries contained so your team can mop up the other team. PS If your new to this game dont buy stackable items such as Leviathan/Mejeis/SOTC, any guide that tells you to get those played against a bunch of scrubs who don't know to jump on a character the second they buy one of those.

Once your level 6 is when you start to destroy. You should either be jungling or at top or bottom. Let the enemy over extend near a tower and you drop your ult with in its range and let the tower chip them down. In a team fight land it on top of Casters or ranged carries, DONT DROP ON A TANK. Ideally you will trap one of your melee allies who can assist in the Butt Rapery. Once you use your ult set your flag outside of the circle in the direction you enemy is going to run. That way if you havent killed them during your ult or if they flash out you can catch and stun them with your lance pull. This gives the rest of your team plenty of time to take care of your target. DONT FORGET at anytime you can lower the walls of your cicle to continue the chase.

DO:
-Get lots of life! I dont suggest warmogs unless you can get it within the first 25 mins of the game. Randuins is the best physical damage resistance. You get a slow and it nerfs auto attackers, and if you get the heart of gold and keep it for more than 5 mins it pays for itself so its basically pure profit to the other items as you go, so pick it up shortly after Wriggles... Which brings me to my next point
-WRIGGLES IS THE BOOBS AND YOU NEED IT. Not only will you farm like crazy, but it helps with dragon kills, and you get a free wards which once again lets this item pay for itself! And the lifesteal will keep you in a fight.
-Build items to counter your enemy. I have a balanced build up top just for reference. If they have more magic damage than physical sub the sunfire for a Spirit Visage or Force of Nature. If they have all physical I suggest the items above. Banshees is good against anyone, along with merc treads but Ill rant about that later.
-Harass at the beginning with your lance and flag. You dont always have to kill them, sometimes just making them retreat for healing is just as important, and generally unless you have a teammate you wont have the chasing ability to keep up. But when you use the lance wait for them to cross through some creeps, you'll end up grabbing a few extra creep kills and you will push to the tower quicker. So you can take out there tower while they are healing.

DONT:
- GET ANY OTHER BOOTS THAN MERC TREADS ON ANY CHARATER. Every team is going to have a stun/slow/silence/knock up. And every team should have atleast one caster so the MR is needed.
- Dont just get life items, if you have 5k life with no resistances your still going to get blown up. Ideally you want 120-150 of either resistance depending on the other team. Hollow life with no resistances will do you no good against any auto attacker or heavy nuker
-Dont get stack items. If you want to win you dont get stackables,even if you get 20 stacks unless you have the greatest team on the face of the earth, the other team will take those stacks away from you. Those items are completely situational and should only be used for a stomping of a bunch of noobs who are playing crit damage nunu...

All of the items and general info ive given can be used towards any hero, especially the tanky characters. Ive been playing LOL since the day beta came out and I own all the heros and a majority of the runes. I understand this game. Every game is situational and you have to play to know what items to get to counter certain champs. Its not dota where everyone gets a sheep stick and starts laughing. If you need more info comment and Ill respond, or go watch Phreaks video on Leagues Youtube.