Maokai is a great support. I think people really underestimate Maokai's ultimate, if used properly along side of aegis of the legion, you could win most of your team fights, of course that's assuming whether or not the opposing team is not fed.

[*] Greater Seal of Resilience: To make it easier to fight that pesky AD Carry early game

[*] Greater Quintessence Resilience: To make it that much easier against the AD Carry

[*] Greater Mark of Fortitude: To help against early-game damage

[*] Greater Glyph of Shielding: To help mid-to-late game damage from the AP

I like to go for "Philosophers Stone", "Heart of Gold" to help sustain in lane longer and for the GP10(Gold per 10 seconds). Magic Treads for that bonus Magic Resist and Tenacity. From there I go for aegis to help my AD carry against damage and to give them that extra damage themselves. I go for the "Shurelya's Reverie" to also help my team sustain with health regen, and if we need to escape really fast the 40% speed buff when activated. "Locket of the Iron Solari" Is also a great item to help with team fights, when activated it gives the surrounding team members damage shield. The other items are pretty much self explanatory.

I chose "Twisted Advance" for the first skill point because it could help you pull of a an early game kill if time and used properly. It will stun the enemy in place for a short duration giving your AD carry a chance to do massive damage, and hopefully pick up the kill. I chose to put 2 skill points earl on "Sapling Toss" to use as a poke. It keeps the enemy away, it can do a nice bit of damage to the AD Carry, and helps my AD Carry farm with limited aggression for the enemies. After I like to max my "Arcane Smash" first for its slow, and its damage against squishies". Then I max "Twisted Advance" for the stun duration.

!!!AS A TANK MAKE SURE YOU INITIATE AND TRY TO TAKE MOST OF THE DAMAGE!!!

*Even if you are 100 gold off from your next item, BUY WARDS. Ever time you go back you should BUY WARDS(unless you either have fresh wards up, or you can can your next item)!

Early-game, if you notice that your lane is being pushed to hard by the enemies, try and keep them at bay with "Sapling Toss". To initiate a fight, you obviously want to make sure you AD Carry is ready. You want to start with your "Vengeful Maelstrom"(you want to make sure that you place it where you think most of the fight will take place), then "Twisted Advance" to make sure your your AD carry has a chance to get to the target and deal heavy damage. I like to use twisted advance to keep the target/s in the maelstrom as long as possible. Right before the enemies are about to leave your maelstrom you deactivate "Vengeful Maelstrom" to deal that extra damage.

Spell Rotation:

1. Vengeful Maelstrom > Twisted Advance > (walk behind the target) > Arcane Smash
or
2. Twisted Advance > (walk behind the target) > Arcane Smash > Vengeful Maelstrom

The reason I chose to go with rotation 1. is because the bonus Armour and Magic Resist is up.

So you need early sustain. Make sure you team is ready(ping if you need to let them know), you have to initiate, and it's in your hands to protect your team for as long as possible.BUY WARDS! I hope this helps.

Remember.. Play Smart, and GL;HF *WARDS WINS GAMES*