Your skill sequence depends on your play style and how the game is going. Each skill has its own pros and cons.

Q: Low mana cost, low cooldown, decent AOE damage. Deciding to Max this first lets you farm your jungle hard.

W: Decent single target damage, never max first but can be maxed second for better ganks due to snare duration, if you feel you have enough AOE damage to clear what you want to clear at the time. Usually you put some mid game points in this if you are a gank heavy player, or leave it until last if you are a farm heavy player.

E: This ability gives you more damage than the other two (assuming you combo the E drop into snare/other cc to get the second chunk of E damage, which every Mao player should be able to do). The problem with this ability is that it has a much higher mana cost than Q and the cooldown is twice as long as Q. You want to put points into this skill when you feel you have the mana to do so... Maxing it up too early will result in you going oom too quickly. If you are a player who rarely farms your jungle and spends the whole game moving between lanes, skilling this up in the early-mid game is more viable. If you plan to farm your jungle hard, just go for Q instead.

R: Get whenever you can.

Conclusion: Personally I like to put points in my E whenever I feel I have the mana to do so, as I like to visit lanes/gank a lot with Mao. After getting a point in each skill I put a few points into Q, providing sustainable clear through the early levels. When I feel that I no longer have mana issues I start maxing my E. E lets you get more damage off when ganking, while also letting you poke and push lanes faster and harder. I like to put a few mid-late game points into my W for snare duration before putting my final points into however many Q upgrades remain.

Mobo boots and Golem are standard on Mao. Sunfire and Spirit Visage are also standard on the current patch. Fist is becoming increasingly popular on Mao, as people are realising just how strong it is on him. Many builds recommend building fist as fifth item, however the point of Fistokai is that you get it nice and early, so you can jump on people and fist them to death well before the late game. The extra damage and crippling slow from the fist makes your ganks way more potent than if you have bought a tanky item instead. Fist also gives you more mana, letting you max your powerful E earlier and throw more saplings. We all know that sapling toss OP.

Fistokai can go in a few directions with his build after your core 3 items (Mobos, Golem, Fist):

-The most standard option is to build into Sunfire and Spirit Visage, then something like Locket or FH as sixth item. You don't really need Randuin's or Rylai's as you have the Fist slow.

-The second option is something I like to do in order to snowball games. I build Ohmwrecker. This is an item that is rarely seen but perfect for Maokai. A well known weakness he has is that it is often too dangerous to use W within enemy flash range of their turrets. While smart players will rarely, if ever be baited into doing this, Ohmwrecker enables you to do it and then wreck them and get out safely. A Fistokai who has got a few ganks off and is becoming huge is already nearly impossible to run away from... Their only defence against it is to hug their turrets like puuussy biiitches. Take this away from them with Ohmwrecker and they have nowhere to run to. The stats on Ohmwrecker are actually quite good value for Mao also... It's like a 2/3 strength Rylai's for 2/3 the price, and you already have a slow at this point so you don't need the Rylai's slow. If you do get Ohmwrecker, make sure you use it well. If you have them scared of you, you can activate it and zone them off their turrets constantly, letting your buddies kill many turrets. If they are not scared of you, keep activating it and doing mini dives on them until they are low enough that they are scared of you. If you buy Ohmwrecker you may end up selling it if you reach full build.

-The third option is to Buy GA fourth and Sunfire + Spirit Visage fifth and sixth. GA gives you the ability to dive a few turrets, while also having some nice tanky stats for fights that occur out in the open. I often buy this item fourth if my team aren't so keen on the constant diving Ohmwrecker strategy.

Also... Buy many wards! From your first or second back onward try buy 1 or 2 wards each time. Personally I like to keep a ward in the bush right beside the enemy wraith camp at all times, as it provides you a fairly constant idea of where their jungler is. This lets you outplay him over and over again ;D

Maokai is usually run 0/21/9 but I believe he is better running 9/21/0. Maokai should be played as an early-mid game gank heavy jungler, not as a farm until late game jungler... So why pick sustain over damage? 9/21/0 is the way to go. Utility tree offers a tiny bit of extra move speed but you have plenty of that with quints and Mobos anyway.

Very standard Maokai Runes.

Quints: Needs the move speed to run people down and get his W off early. Also helps move around the map faster and get more done.

Marks: Magic Pen optimises his clear and gank power.

Seals: Flat armor for jungling.

Glyphs: Here you have the option of going flat or scaling MR. Flat is better if you like counter ganking/fighting early. Scaling is better if you think you can get numbers advantage when ganking, because you won't get hit too hard by champs in the early game as they will be running away from you as you gank.

Maokai is not known for having the most complex mechanics, however that does not mean you can't do some tricky things with him! He is a champion you can have a lot of fun on by thinking outside the box. Your Q, W and E can all be used to play out jukes. Your flash W engage or chase down is always a great time... Be creative. Fistokai is fun times.