OVERVIEW
Jull is most at home in the offlane. In a good lane max Barrels of Fun (this will really boost you stack staking potential), otherwise max To your Health! One level in Bruiser should be enough for the early game. Ask your support to stack ancients for you. You should have To Your Health! maxed at level 9, as it's easily your best skill.
For talents going the right talent tree is a good default build. But if the game is going really good or your team needs more damage go the left tree instead. If you're going the left tree consider buying Echo Saber early.
Jull should be played as an aura carrier. A vanguard is super valuable on him since the damage block works well with To Your Health! A Sange item will also greatly boost his healing. Scepter should be gotten around the mid game, though how early depends on the number of DOT's and slow on the enemy team. Then go the standard aura items. Don't forget to buy some actual damage though, don't exclusively be an aura bot. The shard is a later purchase once your team is more powerful and can make good use of the buffs. Shields are really good on Jull because To Your Health! will still heal whenever a shield takes damage.
Blink is an optional purchase. Jull isn't a good initiator, but if that role needs to be fulfilled then it should be him doing it.