Joker is a great champion. His wave clear is is insane, and his teamfight is really strong.
He has a great escape with his E and can reposition in a fight.

Items and order
So The first item you should get is a medusa mask with the mod to increase the healing when leveling.
Dont max this item out, you should only keep it at the second tier. Then sell it when you need to complete and item or if your out of space to build other items. The medusa mask will give you the most sustain in lane.

Core
The cosmic belt is a very good item , gives a lot of penetration and health.
The second Item is a starheart, being able to put some fire power behind your penetration is a good thing. Starheart is very, very good at doing this.
The third item is Pandoras box, the movement speed and the slow gives you a lot of kiting ability. it timed correctly you can hit a double slow.
Atlatean seal is a good item with the skill harier plus it gives you health.
The cosmic staff is good for the power penetration  and health much like the cosmic belt.
Neron's contract is a great last item! Provides a huge shield and bait potential. If someone gets greedy to kill you and you just nerons right before they kill you, your team might be able to save you or you can outplay your opponent.

Playstyle
Positioning is everything, If you go in to quickly and use all your abilities they will be easily avoided. But if you wait for your team to lockdown and then use your abilities the full effect and damage can be devastating and make the team fight.
be sure to save your e for escapes uinless your posotive it will get you a kill and you can escape from it.
Otherwise it would be smart to save it.

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