This is a very tanky build. You, as a tree, are meant to not move. You will not be doing this. You will be trying to move around the map as much as you are needed to. Like I say in my other builds, I'm not level 30 so, don't bash me or stop reading simply for that fact.

I take a modified caster masteries set of 9-0-21. He's meant to do as much damage as he can without getting extra AP because, as far as I know, the AP scaling on his skills is horrid... but, I might be wrong on that.
He needs the extra cash flow because he has a ton of expensive items and if he tries to jungle for it, he won't be able to do so well until late game.
He needs the extra mana and EXP because if you play him wrong and admittedly, even right, he will run low and out of mana.
He really needs the move speed bonus because he is slow. I can not emphasize this enough. If you can't get away, consider buying the boots sooner.
He really doesn't need the AP boost but, it helps him get the other parts of the caster masteries so.... yea.

That is all.

Sap magic... His passive is one of the reasons you only see him recall for mana reasons, or item buying. Every 5 spells cast near Maokai, be they friend or foe, will add one charge to this. after 5 charges are built up, your next basic attack will heal you for 7% of your max HP. The charges also build up if you cast spells too. Don't forget this. You can also use this as an indicator of whether or not there is someone in the bush using an ability. I'm not quite sure what "close to Maokai" means exactly but, a Janna charging her Howling Gale while I couldn't see her added a charge so, I think the range is pretty far.

Arcane Smash Arcane Smash... One of his big chase spells. A "stun" if they're close enough and a slow. I really don't use this for the slow unless they are really close or, coming towards me at a very uncomfortable speed. It works great for farming due to the slight AoE and line damage. Combined with Twisted Advance and Sapling Toss, it can make for some amazing burst damage.

Twisted Advance... One of your best "support" skills. If there is someone is just within your range, chasing an ally, you Twisted Advance them, drop a sapling on their head, Arcane Smash them and, maybe drop your Ult if your ally came back to fight.

Sapling Toss... This is such a versatile skill. It can be used as a moving/exploding ward. It can help check for enemies in bushes.If they run behind a turret, with low HP, you run just far enough in to nail them with the sapling an probably get a kill. Late game, once everyone has level 2 boots, pretty much everyone can out-pace it sadly.

Vengeful Maelstrom... Such an odd skill. It has it's uses. You don't want to drop it right as the team fight begins unless everyone is grouped together. And I do mean, everyone. You become a slightly less effective fighter while this is on due to it rapidly draining your mana pool. If you see the other team start to leave the circle you lay, pop your ult again. It will do a good bit of damage to them. You may even want to pop it early if someone on their team is close to death.

Take Sapling Toss first to check for early ganks and some nice harassment power. If you are doing a 3v3 and you have some odd people with a notion to just go and get First Blood from bottom lane, you might want Twisted Advance first. If you are having a really bad farming game, you might want to put a couple early points in Sapling Toss. Even though your most damaging skill is Arcane Smash Arcane Smash, it's usually not worth leveling just for that unless you are really trying to be offensive.

I go for the Chalice of Harmony first because he really needs mana. After that, you want to focus on farming minions to rush into a Warmog's Armor. This combined with your passive and your soon to be Force of Nature, will make you one of the thickest and fastest trees out there.
Like I said earlier, if you are having trouble getting away or getting to places, you might want to get the boots earlier than the order says. I only list four items because that's all the game usually lasts for. Everything else is situational.
AD: Buy a Thornmail and just troll people hard-core.
Casters: Just get a Banshee's Veil. It gives you Magic resist and blocks a negative spell.
Other tanks: You shouldn't be fighting them any way. Just leave it.
I would recommend getting a Thornmail in general due to a slight lack in armor. Getting an Atma's Impaler wouldn't be a bad idea either considering how much HP you have. Basically, improve the few weak points of Maokai's defense. If you can fit some AP in there with Archangel's Staff, do it.

I take Heal simply because I can't live without it. If you find a better alternative, by all means, take it. It just helps me lane longer and save some teammates is all.
Teleport because the tree is so slow. I recall. Buy stuff. Teleport back. There is minimal damage done to the turret if any. I also use this for ganks by teleporting to a ward or Teemo's mushrooms or Shaco's jack-in-the-boxes.
Other alternatives:
Ghost: he needs the speed. What more can I say?
Cleanse: He has some difficulty with CC. It's an o.k. replacement for heal.
Clarity: This is a really nice skill for him and I would use it over heal roughly 40% of the time. Heal has saved me more but, I can remember many times where just a little more mana would have gotten me a kill.

Things you shouldn't take worth mentioning:
Revive: You are not a phoenix. Don't act like one.
Smite: You can jungle. You don't need this though.
Ignite: Fire, from a tree? Not really. You might want to take this, ONLY if you plan on being right where the action is, all the time.

Maokai can solo, very effectively too. I recommend that once you feel confident with Maokai that you do try soloing with him once or twice. You really need the EXP and if the other person is squishy/doesn't know your ranges, you may end up with first blood, by yourself, at level two.
The best people to lane against are Master Yi and Janna. I have gotten fist blood on them 9 times out of ten.

There is no one true way to use Maokai. I never follow this build to the letter every single match. Very rarely will I remember what led me to victory last time. So most of it is best judgment. If people are ganking a lot, you might want to use Arcane Smash more for the slow and knock back. If people on your team are kind of slow, you might want to use Twisted Advance more for the root part. It all depends on what you're going up against. And with that, advance not in a straight line but in a twisted coordination.

P.S. Still sort of working on this. did the best I could before I head off to school. Will finish afterwards.

Whenever I make a guide, one of the spells is always heal.... Weird. I bet if I ever make a guide for Taric, it will still have heal.