Maokai, as a general rule of thumb, is simply your basic tank. This build follows as such:
Meki Pendant + Mana/Health pots
Mercury Treads, Unless the other team has no stuns, which is unlikely.
Murillos Evil Tome- Item introduced more or less for Maokai. Gives AP, CDr, and Mana
Banshee's Veil- Eliminates any mana issues and gives you that lovely shield.
Sunfire Cape- Fun fun fun in the sun.
About at this point you should be doing okay, so I usually get:
Abbysal Staff- Close range magic penetration is a boon.
Aegis of the Legion- Your Allies will love you for this.
And there you have it. This build should let you cover alot of ground and leaf your enemies in the dust.
Oh, I forgot: Spell selection.
For Summoner's spells, I go for the great all around Ghost and the utilitastic Exhaust. Cast exhaust on any Physical carry as needed.
Usually, for Maokai's abilities, I go with E,W,then Q, followed up with switching between Q and E, getting R and W as needed. Also, you might want to slap down his R before you go in to stun. This lets your teammates know where you are going and sets the battlefield.