Well you decided to play the character with a hammer bigger than herself. Wicked choice. Poppy can fill a few roles for your team. She can assassinate, initiate, tower dive like a boss, save a dying teammate and set up wonderful intercepts to stop fleeing champions. First I will give you a quick break down of your abilities and why you should love them, then the summoner spells you'll be wanting, the item build, and then play style.
PROS and CONS
+ High burst damage
+ Fast champion
+ Her passive makes her tough to kill
+ Her charge can interrupt channels
+ Six seconds of immunity
- Terrible at farming
- No ranged abilities (Charge places you in melee range, so I don't count it as ranged)
- Requires good teamwork early game to land kills
Valiant Fighter: Please note that this says current health and not total health. This means that the closer you get to dead, the harder it is to hurt you. Perfect for the type of harm I'm going to suggest you put yourself in the way of.
Devastating Blow: This is your "I would like to hurt you now" skill. The damage cap is a little aggravating, but do not be discouraged, this is one of the hardest hitting abilities I've seen and I'm going to tell you how to make it hit even harder.
Paragon of Demacia: This is good for a few reasons. The first being that it gives you more damage. Seeing as how Devastating Blow stacks damage onto your normal hit, this is good. It also gives you armor. Doing the types of things suggested in this build will make you appreciate the extra armor. The best thing about it though is the speed boost and a relatively short cooldown. This will help you escape and initiate your combos.
Heroic Charge: Your initiator! This is the opener for your two hit special. While at its most effective when you hit an opponent into a wall, a well placed heroic charge can also push an enemy back into friendly champions for a quick gang bang before moving to the next victim.
Diplomatic Immunity: This ult is amazing. This will be how you pull of some of the most ridiculous tower dives and should-be-suicidal kills you will ever see. Do not be fooled by the increased damage to the targeted champion. The real beauty is the complete negation of all negative effects from EVERYTHING else including turrets. Be aware that if you kill your target while the spell is still up IT WILL GO AWAY, leaving you in the middle of people that want to put the sharp end of a spear in you, unprotected. The reason I put one point in it at level 6 and don't touch it again until level 17/18 is because I don't use it for the extra damage output. The spell always lasts for 6 seconds regardless of level so I prefer to rush my other skills first to increase my damage output faster.
Teleport: Obviously since you are Poppy you will lack the ability to get creep kills, It's tough but Teleport is mainly for that quick change of location to get that easy kill, or to save a Friend and Turret.
Flash: Flash is for getting to that perfect position to landing a quick pounding on your unfortunate target, and beating a hasty retreat back to safety. Flash has saved me on several occasions. When you start killing frequently people WILL start to target you and you may need to beat a hasty retreat.
This section is the reason I wrote this guide. I've seen to many guides for Poppy that neglect one of the items in LoL that seems like it was made just for her. The Trinity Force is what makes this build work. The reason for this is that the Trinity Force (as well as the Sheen you need to get it) have a nice little buff that adds an extra 150% (Trinity) or 100% (Sheen) base damage to your next attack after spell usage. This is so nice because it stacks with Devastating Blow. So when you Heroic Charge someone into a wall, it primes the extra damage, and then you dish it out with Devastating Blow. The other reason I prefer this build over the AP builds that focus on Lich Bane is that it splits up your damage between AD and AP, but you still hit like a truck. Lich Bane gimps you in AD, making it easier to get items that will help counter you. By using the Trinity Force, you can still get absurdly hard hitting DBs in the form of magic damage, while also having a strong AD auto attack. I just feel like it's a more well rounded approach to dealing pain. This is also why my runes focus on Armor Pen instead of Magic Pen, which I get through talents.
If you want to get kills early it is imperative that you get Sheen as your 2nd Item after boots. The bushes on top/bottom are your best friend by the way. The range on heroic charge is long enough that you can generally pop out and slam someone into the walls surrounding the river before they can run away. Most people seem to like to hug that wall because they want to stay away from the bushes, but in your case they're doing you a favor. The boots are all preference really, I think the extra speed from Boots of Swiftness is needed on Poppy, As it makes you run extremely fast when you have W Popped. Once you've got your boots and sheen, finish up your Trinity Force. I personally recommend getting Phage after sheen before zeal, As it gives you HP for survivability and slows your target. You should be able to get enough kills to supply you gold to build it all fairly quick anyway. So onto the next section.
So if you read the first paragraph of my guide you will have noticed that I never mentioned that Poppy will be tanking with this build. There's a reason for that. If you try to be the front man for your team and take the hits, you are going to die. A lot. Let your tank do his job or let the enemy team make a mistake. Be patient and wait for a good setup! If you see an enemy charging up a skill (Fiddles ult, Nunus ult, Warwick just stunned someone with his) do your teammates a favor and charge them! You are an excellent channel interrupter so use this to your advantage. Try and stay in the middle of your team so that if someone comes from any side you will have a better shot of hitting them into a wall.
Let's say that you were not there when the fight started. Do you charge in from the front and just start swinging? I think not. This is an excellent opportunity to use all those paths in the jungle to get behind your enemy. Ping the target you are planning on attacking so your team knows to be ready to kill someone and then charge full speed ahead from behind and use Heroic Charge to push your target into your team. If you can hit them into a wall as well then do it, because then they're screwed. If you can't line that up, just be satisfied with hitting them into your team. It should still be sufficient to end someone. Every tower in the game has a path to get behind it (excluding towers behind the base wall). Use this to your advantage. Do the same thing just described, only make sure to use your Diplomatic Immunity so the tower doesn't eat your face.
Diplomatic Immunity is strongest when it is used more for keeping you alive in the middle of the enemy to kill a high priority target and escaping afterwards. The key to that is putting it on a champion least likely to harm you. A caster who is out of mana? Perfect.
My last bit of advice for you is this: GET THE GOLEM BUFF! With some cooldown reduction runes and the golem buff this will put Devastating Blow UNDER 3 seconds. That is absolutely absurd considering the damage output of this skill. GET IT!
Those are the basics to successful Poppy play! Remember, don't be afraid to tower dive, don't be afraid to initiate with a Heroic Charge from behind into your team, and enjoy your face smashing fun.
This is the first guide I have ever written for any game