Firstly, I think Poppy is a game saver. She is one of my favorite carries because she deals a lot of damage over a short amount of time(DPS). She doesn't know any jokes, but she sure does make the other team look silly if you use her right. She constantly dominates with her easy Heroic Charge to her Devastating Blow. And by end game she kills very easily. The beginning you are gonna want a ranged or good farmer because Poppy is awful in that department. I do extremely well with healers like Soraka and Sona. The hardest part with Poppy is hitting people into walls. Its very important and you can save many teammates this way. Secondly, Feel free to be greedy! Her ultimate Diplomatic Immunity is amazing. Generally other champions don't understand that they cant damage you if you target someone else so they'll waste their attacks on you who are in the middle of the fray while your teammates wipe them out. The most important thing to remember is that you can't always do it alone. I have seen way too many Poppy's heroic charge into enemies and get annihilated by everybody and their uncle out there. Communicate with your team before you make rash decision. League of Legends is a team game and Poppy is TEAM player. Remember: there is no Team in Poppy, but there may be a Poppy in team. Haha. This is my Poppy Build, enjoy. Give me some feedback and let me know that i suck and are awful at making guides.
Also: thanks to Dotter for some input to help this guide <3<3<3

WARDS! I play mainly 3v3s and i have wards on dragon and red elder lizard to show incoming ganks and they are the main buffs for the map!

August 20, 2011: -Changed up the item sequence a bit to add more AS and MS with Phantom Dancer
-added a 3v3 spotlight column
-added wards into introduction and fixed some icon issues

October 25, 2011 -Added Nashor's Tooth for AS.
-New (Runes)Seal's for more Armor
-change up in summoner spell and masteries

Pro:
Great AD
Great DPS
Good Movement speed
Great abilities
Good team player
Can gank well solo
Jungle buffs help tremendously


Cons:
Squishy early game
bad farmer
Needs to be fed to dominate
E ability requires good timing and aiming

Start game: I recommend a side lane for obvious reasons. I prefer to go with a ranged character or healer. draw in greedy players and put them in the wall until you have enough for sheen. Then make your boots and hunt down those squishy enemies.

Mid: you have just finished your trinity force and you have your mercury treads. You're turret diving like crazy and the enemies are crying in game chat about how much of a noob you are. Now you can solo the red lizard and take his buff which makes u slow enemies with each hit. You may even want the blue golem's cool down buff as well.

Late: your team is pouring into their base, but they can't get through those pesky nexus towers. So you use diplomatic immunity and get a double kill now that you have your infinity edge. The other team is trying to surrender but that one kid just won't vote yes. Then you win. Woo hoo! Yee Haw!

Devastating Blow
Huge damage dealer. All squishy champions should fear this. The best way to use them ability is to first charge with the E ability into a wall. Enemy champion is now stunned and crying to their momma. Then comes Devastating blow. Owchy. its limit is no big deal, it hurts.
Paragon of Demacia
I use this rarely. I use this W ability to hunt people down, to run, or when I'm up in someones business and i need some armor. Its useful to say the least. It makes you a mini tank in those 5v5 battles in mid. Use this ability with ghost and exhaust to really hunt down those scared enemies.
HEROIC CHARGE
This is your first line of attack. Get someone in north brush and put them in the wall. It will stun them and leave them helpless. This can also be used to save a teammate who is about to be finished off. This is the first part of the 2 Hit KO in late game. Use it wisely.

Diplomatic Immunity
This is the best ultimate i have seen thus far. You deal extra damage to target AND you can't be hit by other champions other than the one you targeted. This is a turret dive champion right here. But be careful, if you kill the champion the shield drops. Use Ghost to get out of there. I use this ability for damage mainly so i upgrade it as soon as i can, but it can also be very useful to get out of danger. Throw it on a melee champion far away and run.

Ghost or Flash AND Exhaust

Ghost: useful to hunt people down or to run away from those Demacia to really get that movement speed up.

Flash: same deal here get somewhere quickly

Exhaust: slow them down so you can get a lot of devastating blows on an enemy champion and allows your team to gang up on them.

Poppy is definitely an attack damage(AD) character. You can either play an attack speed Poppy or a larger AD Poppy. Her DPS is naturally ridiculous because of her OP abilities. But it is the items that are the icing on the cake of justice. Firstly, the Trinity force and sheen are a must. Sheen allows you to slam enemies into walls and then do additional damage on your next attack. Trinity force has everything you need. Self explanatory. Mercury treads help you run through slows and stuns. If you want to be an Attack speed poppy get the greaves. But, after getting the trinity force you need to read the enemy team. If I'm getting pounded by mages ill buy a Banshee veil . Or if I'm getting killed by AD ill buy a Sunfire cape or other armor item. Once you are doing well you need to start building AD. Buy the sword and get the black cleaver and blood thrister if you want. Most of the time you will not get to the infinity edge . But it will dominate if you can get it. Also, you will notice that i Don't have lifesteal. I don't think its as important. I you MUST have it get a blood thirster after your black cleaver.

Edit: If I have more than 3 deaths by time i get enough to buy the Zeal or the Phage i always buy the Phage first. It doesn't matter either way. If you need health get the Phage, but if you want to continue humiliating kids buy the Zeal for Movement speed and Attack Speed.

This is just another section to recount everything I've said. You're job is to have map awareness and gank enemies when possible once you have you're Ult and trinity force. You can push enemies back towards your slower allies and let them stun them. You need a VERY good laning partner. Preferably a good friend you know or someone you know you can rely on.

Ok, for 3v3s i generally play with a tanky character and either another AD Cary or AP Carry. For example ill play with Maokai or Alistar (mainly as a bot lane partner for the knock back) or Janna and Master Yi .
These partners can take damage and most have a stun or knock up like Janna's whirlwind or Alistar's slam. For 3v3s you take charge as team killer/carry. GO FOR THE SQUISHIEST CHAMPION FIRST IN ALL TEAM FIGHTS! If you and your team are face to face with the opposing team of; For Example, Master Yi, Lux, and Twitch, Get twitch first then the other two will probably be taking heavy damage/disables from your team and will b easy pickings. You want a lot of kills. Get the buffs especially dragon, and wards will help you. 3v3s are all about the kills and getting the squishiest down first. You need good team communication when it comes to ganks and mias.

Poppy needs to be played a certain way at certain times. Here are some common situations and what i would do.

Early PvP. You and your partner are exploring the brush on the top lane. they wait for u and attack you. But they are foolish because they pretty much just gave you a wall for them to slam into. It'll give you time to run and let your hopefully ranged partner weaken them.

Those 5v5 mid confrontations: Don't be afraid to use they ultimate on the foe with the least HP and run straight at him and try to get the kill. Be careful because you can run out of Ult quickly and will get ganged up on. Don't be afraid to be the semi tank with your Ult.

jungle battles: this should be your favorite Poppy fights because walls are everywhere. It's fun to gang up on greedy enemies or tear a team apart.

Turret hugging/ ganking: how are these similar? Well if your team is ganking or you are turret hugging you have the option to get behind an enemy and push them into the turret. It's a great way to assist allies who are struggling by their turret and can't reach champions like Ashe. Just slam here into the turret and give her a Devastating blow to clean her clock. The turret may do the rest.

Winning: you own the enemy base. You can push enemies into their inhibitors, their turrets, EVEN THEIR NEXUS. just don't go pushing to the platform turret. That can never end well.

Poppy is the daddy of the map. Never forget that. She controls the team fights with her extreme damage that will split the enemy force and send them running. Again a healer or ranged like veigar in your lane with a stun is very good. Poppy is kind of squishy in the beginning until the Trinity Force. Go get 'em