Not Updated For Current Season
So basically all you need for a good Maokai is a lot of AP and quite a bit of health (the health is for his passive).
As far as runes go, I like to start with all mark and quints as straight up AP. This gives you a bout 20 or so AP off the bat. The rest of the runes are Glyph cool down, and more AP seals.
I prefer to start with amplifying tome to give extra AP. I also pick up a mana potion (Maokai is mana hungry). As soon as I have the cash, I work my amplifying tome into a Will of the Ancients (yes, I'm aware Maokai's passive gives him life back, but this has saved me multiple times). I then follow up with a pair of Sorcerer's shoes. Right after the boots, I get a Crystal Scepter for the health and slow. Then comes a Deathcap. Then a void staff. And finally either a Rod of Ages or a Zhona's, depending on how the game is going.
You can pick up any kind of spells. I like to take a clarity because I spam my sapling toss, and depending on the game, a heal or a ghost.