Not Updated For Current Season
You may know and love Maokai for that big tree guy who lobs grenades... but he has 3 other skills, plus his passive, that require him to get into the direct range of any champion. Tank Maokai lets him use those skills aggressively to annoy your enemies and benefit your team with more than just damage... something much, much worse: Crowd Control!
Playing Maokai as tank means:
Initiating, maximising team survival.
Saving your squishies, pinning down enemy squishies.
Planting some Wards, stealing some buffs.
Die a few times, get fewer kills.
Be part of every teamfight, get every assist.
Tank the damage, support the team.
Tanking is rather simple.
The amount of damage you can take is relative to your resistances and your health.
Every point of resistance grants +1% effective HP (EHP) while every point of health lets you take an extra 1 point of damage. Simple.
This is because the amount resistance is = ( resistance/(resistance+100) ) * 100
If you have 0 resistance, you take 0/100 * 100 = 0% resist, 100% damage taken means you take 100% damage to die.
If you have 100 resistance, you take 100/200 * 100 = 50% resist, 50% damage taken means you take 200% damage to die.
If you have 200 resistance, you take 200/300 * 100 = 66% resist, 33% damage taken means you take 300% damage to die.
So, when you're looking at your items and thinking "what's should I buy?", loosely calculate the benefits. For example:
"If I buy a Null-Magic Mantle I gain 24 MR, 24/50 = ~50% extra EHP vs magic..."
"If I buy a Ruby Crystal, I gave 180HP, 180/757 = ~25% extra HP vs everything..."
"I should buy the mantle because 50%>25%, and my enemies are mages!"
Similar processes go with attack speed, armour penetration, critical strike and damage for DPS or AP, cooldown reduction and magic penetration for Mages.
Maokai / Tank / Support Basics
Balance the lanes - Never lane with another tank or support, you will have little or way to retaliate to harassment.
Don't be the solo lane - You will have a hard time being solo, and you being lvl 6 before everyone else DOES NOT help! Let someone who can Midlane good take the solo lane.
Diversify skills - Try to lane with someone who has not got a root, stun or scare.
Nourish your allies - Try to make sure your laning partner gets at least half of the minion kills.
Feed your carries - Try not to get a kill when your teammates can; if you get a killing spree you will fund the enemy when you die for the team.
Better me than you - block skill shots that'll hit your teammates, if your teammate is low on HP walk over him with your big hitbox to prevent people clicking them. Take the damage they can't. Make your squishies keep their stacks and you will keep winning the game.
Knowledge is power - Use your sapling to check bushes, ward river, ward dragon, ward baron, ward buffs. Then steal their buffs after their hard work! Guide for warding:: www.leagueoflegends.com/board/showthread.php?t=237514
Call Missing - If someone goes away they might be at shop... or they could be stealing your blue buff or ganking your mid!!! This is important early game around 5-30 minutes. Say "ss <champion name>" when they go missing and "re <champion name>" when you see them back.
Expect the Gank - Use those sapling senses... it's 6 minutes in and no one's seen the enemy jungler... ward river with my sapling or check the buff on my lane for them! You will make them paranoid for the rest of the game.
Ping - Use G to ping. Ping anything of significant importance. If an enemy is coming to gank then ping them. If you are coming to gank, ping who to focus. Ping home to defend, ping turrets to push. Ping Shaco or LeBlanc after they duplicate to pin the real one.
Prioritise Targets - don't chase after the tank. Go after the DPS or Caster. Focus fire using G to tell which person to go for first. Look for the weakest person with the greatest damage output. If someone has more than 10 stacks on Occult or Mejai's then team up to take that person down when they are out of position.
Fair Trade - Dying should be avoided where possible but it isn't always bad; if you took all the burst damage from the enemies and died but that allowed your team to kill 2-5 of them then you did a GREAT job. You might not get any assists but YOU won that fight.
You're a tank. You build respective to your enemies. I've put in the sturdy "core" of my Maokai preference which is fairly balanced and adequate in price. Note that you won't get much gold throughout the game, unless it goes on for a good duration. 6 Items? Game has gone on for far too long.
When you soloQ tanking is normally easy. People generally pick squishies which boils down to either all AP or all DPS with one tank (maybe). Against more veteran enemies you might have to balance your defences more thoroughly. Consequently: you will be a valued rarity playing the tank and be a huge credit to the team's success.
Doran's Shield is for holding your lane stronger. The HP boost builds with your resistance, and the HP/5 helps you dominate your lane a little better.
Going for Mercury's Treads is a must, you simply won't help if you're taunted or slowed or stunned and can't use your abilities to aid your teammates.
Following up with Aegis of the Legion benefits you and your team. You gain balanced defences and fair HP. The +8 damage also pours onto your minions, which is a nice boost for pushing towers. It even stacks with other champions.
I find the team composition usually boils down to: 1 Tank, 1 Support, 1or2 DPS, 1or2 AP. (+Jungler ideally)
2 + DPS heroes: Build Armour
* Attack Speed: Warden's Mail for armour and slow passive. Follow to Randuin's Omen.
* Skill/Damage: Sunfire Cape for HP, armour and burn passive.
* Bloodrazer's: Warden's Mail + Frozen Heart
Following into Force of Nature for the MR+Passive.
2 + AP heroes: Build Magic Resist
* Core: Force of Nature has ~100HP/5 but less MR than Sash+Chalice.
* You have healer: Spirit Visage as it'll buff heals from your passive too.
* Stuns/Cheap: Quicksilver Sash and Chalice of Harmony offer a fast solution to MR.
* Rich: Banshee's Veil for HP.
If you find you have enough MR, go into Sunfire Cape or Randiun's Omen for HP+Armour.
Hybrid or mixed heroes: Build Health
* Build Sunfire Cape for HP+Armour.
* Build Banshee's Veil or Force of Nature for HP+MR+Passive.
Prioritise the items depending on the enemy team composition
Those Other Items
I haven't tried all of these other items, I see them on other builds though.
Thornmail: +100 Armour and 30% reflect damage sounds good but you will only deal around 10-25% of that on an enemy. It also only works for you. While Frozen Heart reduces all nearby DPS by 20% or Warden's Mail which reduces DPS+Movement by 35% I feel both are superior versus DPS characters.
Catalyst the Protector
Early Catalyst: this is not a bad choice, opening into Banshee's later on. I would choose this if you are getting pushed back on your lane so the passive can benefit you.
Rod of Ages: now this is one of those "tank AP" items, but you're not Singed. although the 800HP is a lot the Rod of Ages costs a lot and provides no resistances at all. The AP will add 40-100 damage on your skills but you are NOT the damage dealer of the game.
Soul Shroud: for 500hp and a nice aura, it isn't bad for it's price. If you find yourself with mana hungry allies then this isn't a bad choice. My problem with it is that it grants no defences like Rod of Ages and a blue buff negates most/all mana issues.
Rylai's Crystal Scepter: HP, AP and slow. Like Rod of Ages, no defences. The slow will barely work either: your Q+E+R are multi-target skills (15% slow) so your Q slows for more, while your W lasts longer than 1.5s. Hitting with E on a running target is also quite hard. A Giant's Belt and Morello would be far better.
Warmog's Armor: like rod of ages, for me it is just too expensive to set up. End game it's about +40% HP but compared to picking your resistances right you're worse off.
Guardian Angel: not a personal preference but is a good all round tanky item, letting you initiate without worry. However, I find when someone has their guardian angel up they are not attacked as much which completely negates the ideal of tanking.
Skill Leveling Sequence
to level 1 first, then maxed last
Level E, Sapling.
Free wards, check.
Use Saplings in the well to get 3 stacks on your Passive then go to your lane and use them to check the bush on river and the bush on the side of the map to check for that initial ambush.
After that Focus W, Roots, and Q, Smash, equally to maximise your control over players. Getting your ultimate whenever it's available.
Skill Combo Sequence
R where required.
Release passive whenever possible.
Try to keep your mana as high as possible, save it for players. Harass early with your sapling but mainly use it to ward where required. Keeping sapling at level 1 minimises the mana cost of it meaning you can be a little more frequent than others.
When early battles commence open with W on the squishy and throw your E on top of them straight after. When roots wears off use Q to knock them up, holding them for even longer. If you have the health, the safety and the backup then chase them down a little. Otherwise back off to regenerate mana and HP.
The same combo applies later on but you have to be careful not to W yourself into 5 people.
Use R when you have a good sum of mana, even if it's only used for 5-10s it makes a big difference. It's on low cooldown regardless. Combined with Aegis you will give your team a good sum of damage resistance, it is effectively the reverse of Vlad's ultimate.
You can use R when taking on Baron or Drake as well.
Flash - It lets you get in or out of sticky situations. Sometimes you'll use your root on an enemy and he'll start running away at the speed of sound (Master Yi) dragging you into the middle of hell. Using flash lets you get out of these spots. Flash also lets you chase down and root champions with Flash Abilities like Shaco, Ezreal or Kassadin.
Improved Exhaust - In my experience not many people seem to pick this up. It lets your pin down enemies and effectively grants +10 Magic and Armor penetration, great for focusing down on a specific hero.
I wouldn't recommend losing improved exhaust, due to the mastery fitting in there, but changing out flash for Ghost, Teleport, Cleanse or even Clarity will help you in whichever area you feel needs attention.
Ghost - For Chasing down or running away
Teleport - for greater map control and tower defending
Cleanse - You could get a Quicksilver Sash instead
Clarity - Mana was an issue for me but just get stingy with your skills, save them for players.
If you are unable to purchase T3 runes because of level or cost, just get T1 instead. They are cheap and effective. T2 are not worth the IP.
Armor+MR runes are cheap and are great on any character. Getting them adds a nice boost both your Armour+MR, giving you a starting boost of 19 which cancels out both Sorcerer's Treads and Brutalizer. Congratulations, you just made your enemy waste 1100-1350 gold!
MR or Armor marks are great, personal preference is MR because Maokai doesn't have a scaling base MR.
HP Quints... expensive... but very worth it! As soon as you get the core they've doubled in effectiveness, +156hp! When you are with your stronger resistance they are worth about 250hp. Combined with Doran Shield and masteries your total initial HP boost is 246! Which values up to 750HP late game. You also start with a nice 757 HP.
Bane of your existance
Some champions are made to kill you.
Some items are made to rip apart your defences
Sometimes your team are actually noobs.
Sometimes the enemy team is just better.
Oh well! You'll respawn soon enough. Accept the loss with a GG (assuming no one left on your team :P)