The best part of any good Marco player is the play style. Stay away from the fight early on and poke with his Q (Poison Dagger) by throwing it at the nearest target. It is still useful to throw your Q on a tank because there is a chance that it will jump to another target and deal more damage. Remember to throw the Q as soon as it comes off of cooldown to harass the enemy. Even if you don't kill them, making them go back to base for a heal allows your team to push.

The next part takes some eyeballing skill. His E (the jump attack) will deal a ton of damage to low health targets, so if you do it right you can instakill them. However, you have to be careful of what I call "the line". If you jump a target behind two full health champions (especially if they deal alot of damage), you will almost always die. Only jump a target behind "the line" if you have an escape route or team mates that can keep the other enemies busy. In sticky situations, activate his W (damage reduction shield) to aid in getaways.

For Poison Dagger I maxed out damage first, that extra tick is life or death. After that I got jump distance which increases the range it will jump to another target. For the W Shield I alternated healing and damage reduction but maxed on healing because the final tick will heal you for a portion of your max health. Finally for Death From Above I maxed out cooldown immediately, which refreshes the ability on kill or assist. For this one, I went with the points in Area because it doesn't heal that much with the Lifesteal points.
As a general side note, masteries points can't be spread out over all three attributes, the game forces you to pick one to max out and spend two points in the other. So which ever two you choose, be ok with having to spend all your points on it. Also, reallocations cost 5000 points for each ability which sucks but it pays for itself in about 10 victories in pvp.

Some Marco players make the mistake of building him pro-Void which leaves him very squishy if you have bad team mates. My dynamic build can help you win any match even if you get a terrible team mate that feeds. As with all builds, find the one that works for your specific play style. Start with Barbed Stiletto (Off-hand, Daggers) and build straight to it's upgrade, Death's Shiv. If you already seem to have bad team mates, I'd suggest going with Steel Stompers (Iron Boots) but if your still doing ok then go with the regular Void Walkers. After that get Scimitar (Main Hand, Swords) and one of it's upgrades. If they have a healer Support, get Foul Play, but if not get Starshard for insane damage to turrets.

Now this second part is where it gets fun. If you're doing mediocre or have bad team mates, get Armor of the Immortal (Chest, Armors) for crazy saves in sticky situations. This item is especially a great counter to damage over time effects that would normally kill you. However if you're fed, get Vitriol Cloak (Chest, Vests) which returns a portion of the damage to your attacker. This makes diving kills much less risky because they won't want to attack you. At this point if you are dying alot, get Lunatic Guise (Head, Hoods) which reduces respawn time. If you're still doing good go with Executioner's Hood to get a heal on your abilities. Finally for an accessory, you could really go with any of the Belts because all of them have wonderful attributes. My personal favorite is Nothing Left to Loose, which makes you immune to all snares INCLUDING THE WATER. So very useful for some crazy escapes and countering any crowd control characters.

A great way to get this build memorized quickly is to start out buying the recommended build, but navigate the shop to get the items instead of just pressing auto-buy. This way you can familiarize yourself with the shop mechanics, the items order, and different options that you have for a build. Remember, you can switch up the order that you get these items in depending on how the game is going. However, I always get Death's Shiv immediately followed by boots because speed is everything in this game.