Early on Poppy is a decently strong fighter against most melee matchups. You want to start with a tear and 2 pots. You want to trade with your Q into melee matchups as much as possible, stacking tear and dealing damage. Since her Q is decently low cooldown and aoe, you can use it to push waves easily. In an ideal trade you are killing minions and hitting the enemy with Q at the same time. Whenever your passive is up, you want to throw it at the enemy champion most of the time. Unless you are using it confirm a minion kill or the enemy is too far away to safely use, always throw the shield at the enemy for poke, ideally you throw it when grasp of the undying is charged up. After throwing passive you want to collect the shield to reduce incoming minion damage or for a trade. Sometimes the shield might bounce to an annoying position, if this happens you can do one of two things: ignore it or make an enemy think you want to pick it up which forces them to move towards it, then just hit them with Q for free damage.

Poppy does much better in to AD matchups due to items being more favourable. Into a high magic damage lane you will often forced to be more passive.

On first back you want to buy tier 1 boots. After boots you can either start building into your mythic or into thornmail, depending on your matchup. The only time you won't do this is into high magic damage lanes like teemo or rumble, in which case you want to spec into magic resist, spectre's cowl or negatron cloak.

Poppy can set up rather easily for ganks if you are being pressured, but you need to be ready to take a hit or two to bait the enemy. Poppy is deceptively resilient and with her passive you can turn many fights for the better. If your jungler is looking to gank you, you might want to trade aggressively, purpose taking damage and doing as much as possible without burning cooldowns. If the enemy takes the bait you want to play around your passive, prioritising picking up the shield. If you are are pushing the enemy in when trying to bait for this gank, try and position near one of the walls so you can stun the enemy when your jungler arrives. You want to be ready to use your W to prevent an enemy escape too. If all goes well, your jungler just picked up a kill and you are still alive. Poppy is also good at 2v2 skirmishes so always move to help your jungler if they are fighting in river if you can.

Short of a disaster, by the time turret plates fall you should have your first item. If you do not, do not worry, poppy is very hardy champion. You can switch to being a peeling support or sacrifice for your team as needed until you get three items.
If you are even or ahead, you generally want to stay in lane until you kill the enemy tier 1 turret. You should only leave the lane for big teamfights.

At this point a few things could happening.

You are miles ahead:
If this is the case, its likely you can take on three enemies at once and either kill them or get away without dying. This makes you great at tying up enemy resources. If other enemies are ahead as well you can be a very good meatshield for your team and disrupt the enemy and protect your carries.

You are even:
You can confidently 1v1 your laner or another duelist and and distract them from your allies. You might struggle against unfair numbers like 1v2 or 1v3 though so try not to misposition nd get caught out. In teamfights you are going to tilt the **** out of the enemy with your W and hunting down threats to stun them with your E. You can turn a fight with your W if timed properly, or with your R. If you can use it right, you might be able to turn a 5v5 until a 5v3 or better, allowing your team to clean up whos left. You can also use it to save your team if you are losing the 5v5 in the same way.

You are behind:
You will not be able to do much alone at this point, but you can still be useful in a teamfight. Do not worry if you are behind, just farm and continue to play. Once you get to three items (as long as one of those three is fimbulwinter) you will be back in the game. Fimbulwinter gives you so much agency its unfair and you can tank so much more stuff. If nothing else you will always be a great asset for burning enemy cooldowns, allowing your team to win in teamfights when enemy cooldowns are down.

If you are on the verge of losing a game your ult can be a great tool for defending the base. Since it can hit minions and champs you want to use it to knock away as possible to buy your team time. Your tankiness should also help in trying to hold back the enemy.

If you have built tank, you will not be very useful in sieging the enemy base. You can push waves really easily which is good but your ability to take towers and inhibs is really bad so don't try to base race or back door unless you have a huge amount of minions. You are great at stopping enemies from depushing waves and protecting your team if they are sieging.
If you have built more of a bruiser build, sunderer will help in taking structures and you can actually be a backdoor threat but you will be outperformed by other spltitpushers. So do not try to bace race a split pusher like yorick or something but if the enemy team doesn't have one you can do it and be a threat the enemy has to deal with or lose to.

You are an annoying tanky problem the enemy has to deal with. Against some teams you can be a damage carry, where as against others you can just be a shield for your carries. But you are always a force that tilts the enemy.

Do not be afraid to take a few hits to get something done.
Need to bait the enemy? Take a few hits to make them think you are killable, then ruin their day with your passive and fimbulwinter
Enemy assassins and bruisers are chasing your carries? Your life is expendable! Get in there! Stun the enemy or knock them away with R! Protect your draven from a leaping rengar with W! There is a high chance you might actually survive anyway so don't hesistate to put your life on the line unless there is absolutely ZERO chance the carry gets away.

Lastly, very important!
NEVER SURRENDER! EVEN BEHIND YOU ARE A MENACE! THE GAME IS NOT OVER (most of the time) UNTIL THE VERY END!

SHOW THEM THE HAMMER!