Poppy is underused and overpowered IMO. but heres the easy guide to winning
The first thing you want to do Is take a lane with either a support or a tank. Early game unless your opponents are short bus riders will demolish you. farm your creeps for about 1000g or untill you are lvl 6.

At 6 grab at least your boots and get back to laning.

right around 8 or 9th level your going to be able to rack up more kills. abuse your stun as much as you can and if your opponent manages to escape pop your ult AS SOON as your in their turred range. NOT any sooner. ever, If they have backup and your under half hp just let them go.

At level 11 you'll finally be able to rack up multikills. it your in a team fight and drawing the most fire pop your ult on the opponent thats doing the least damage that DOES NOT have a stun ability. and attack the opponent that is doing the most damage. this seems like an awkward plan at first but the extra damage on one opponent will not compensate for the double kill you could get. with your ult active the only thing in the game that can harm you in any way is your target. BUT you can still beat the hell out of anything in your way.

late game is when poppy shines the most. your going to have enough speed to chase ANYONE, even if they ghost or flash. after phantom dancer you will have 540 move speed while W is active and extra range with charge. Poppy can be the biggest asset to a team when she gets targeted by multipliable opponents. that is the perfect time to use Immunity. giving your teamates 8 seconds to beat down everyone trying to beat you down (and failing)