I'm going to ignore the guys above me , but i'll be honest to say that I haven't played Poppy since 2015 March ect~
but how I used to build her is I'd focus Triforce first, and focus on Speed boots with aclarity, hydra is another great item it elminates poppy's push weakness. usually Speed boots Triforce and Hydra are my main items I aim for the other 3 slots are sorta a mystery I usually like to go full Damage
- True Damage is Poppy's biggest weakness that u should be aware of.
- Armor/MR/Health doesn't benifit Poppy's passive
- Mana Regain isn't worth anything because think of Mana as being Poppy's killing license when its out you should just go home.
Poppy can do absolutely horrible in lane and still do amazing if she makes an appearence in another lane and picks up kills, actually her top lane opponent is 99/100 going to counter poppy.
sometimes you can also use your ult as a scare tactic say if you ulted Ezreal and killed him like 2-3 tmes already later on you ult him he'll run, and you can beat on his team invinsible.
I'm no pro i'm someone who just stalks her rework forum since she's one of the few champs i do well with.