Well, Hallo der!!! I see you wanna be a tree. Great! First things first. This is how I play Maokai and how I play in LoL. It may not work for you or it may just not be the Champion for you. So, if that's the case, don't down vote because of that!

Pros

Really fun
Is a tree
Great CC
Helpful in a team fight.

Cons

If caught alone, kind of easy to kill.
Just a tad on the slow side.

Let's see here....
9/21/0 I think is best. The spell pen in the offensive tree is really needed because if you
don't go an offensive spec early, your damage output isn't that great.

0/21/9 This is used if you strictly want to be a support tank.

There are rooms for other masteries, but I don't think there all that great so if you want a 0/9/21 build go ahead. If it works, even better! =D

Okay. I know exhaust and heal may seem a lil noobish, however they work. If you dont agree, I'd suggest either getting clarity and changing around the mastery a bit so you dont waste points. Also, another good one would be teleport.

I'd say runes are pretty important for Maokai.
Marks- The magic pen is really nice and can make up for the low damage since he is a support tank.
Seals-Mp5 is a must. Maokai is mana dependent. In stead of building flat mana which is crazy Mp5 is where it's at.
Glyphs- The magic resist is nice for when the items don't cover the rest.
Quints- The health is nice. I mixed the two because the two flats at the beginning help a bit and the scaled one for a bit more once your 18.

Okay, this section is very situational. Get your saps right off the bat. Then from there, I usually play a more defensive/support Maokai. Hence my mix of Twisted Advance and Arcane Smash.
If you want to play an offensive, get more Sap toss early game.

I really love this item list. The last three are up to you. Wanna move fast and have a bit more health regen/mana resist? Force of Nature. Want a little more umph? Void Staff. Wanna be a die hard tank? Randuin's Omen. I usually get the Omen but have caught myself getting Nature due to some magic based teams. And Sunfire Cape on a tree just looks hilarious!

Early Game
So, grab your awesome Sapphire Crystal and some pots(insert plant joke here) and head off with your lane partner and get ready for some fun harassing. Throw your saplings in some bushes and watch them hunt down the enemies. After you've had some fun, you can mosey on back to your base and get your Catalyst and some boots! Now, the boots are situational. If your enemies have some CC, Merc treads all the way. If not, swiftness! Grab some more pots just in case and head back out there!

Mid Game
Now it gets a bit more fun. Farm minions using all your skills since they have such a low CD but save some mana for team fights. Your stun/snare should at least be about 2 seconds now. Head into a team fight, and Twisted Advance to the unlucky champion. If there team is there and yours, lay down your Ulti. Then walk around and Q the champion and hold him a bit longer. If he seems he wants to leave turn off your Ult and toss a sap at him. Strut back to base and buy some awesome gear with the kills/assists you just got!

Late Game
This is where it gets really fun. By now you should have 4 1/2 of your items if not more. You get hard to kill. Just walk all in a team fight and be like WHABAM! Disrupt everything. Lay down your Ult and go to town. When things have settled down, push on down a lane and easily farm some minions. Spam your CDs and get monies to buy the last of your items. Remember, your last item is situational! =D

So get out there, knock a few champs around and laugh at your saplings go! I hope you love Maokai as much as I do. And if this build works for you, thats as great as an oak. Hey, c'mon... I had to add atleast one sort of tree pun in here... Sees ya!!!