Not Updated For Current Season
These are my two builds that i have thought would probably be the best for Maokai. I will be talking about how to maybe do some changes items/rune wise that you can do. But over all this is going to be pretty straight forward.
E: Get E (Sapling Toss) first and max it out first. Its good for scouting before minions spawn as well as harassing in a lane if you can land it correctly.
W: Getting W would be next, make sure to get it at level 2 for that nice root in place. As well as the fact that your W is your key initiation ability.
Q: Q would be maxed last, doesn't do amazing damage. Its key thing is to interrupt/delay enemies.
R: This spell can be difficult for some people. When to turn it on that is, and at what position. It takes some practice and some studying of the situation. But when understood- it can change the tide of a fight.
I would recommend Flash/Teleport for your first Summoner Spell. Flash is great when you use it to get in range for your W. Followed up by an Ashe arrow or walking around them to use your Q can be a great way to start a fight in your favor. (Keep in mind that the game is situational, so you need to pay attention and know if your team is ready for what you are about to do.)
For your second Summoner Spell i would get Ghost. Maokai, the big tree that he his comes with slow movement speed. Ghost is an overall good spell for chasing someone to trap them with your W, or to even escape.
Spells i would NOT recommend getting:
Smite (unless jungling)
Clarity (you really don't need this if you use your skills wisely and do not spam.)
I have two ways of building Maokai. One is purely tank but i will explain why for the first one you probably want to get Doran's Ring. And the second one has some AP for a little DPS and mana if needed, as well as some more magic pen.
KEEP IN MIND: Before i explain what i get, please keep in mind that items are situational, you might need to get ninja tabi, more MR, more Armor, HP. It all depends on who they have.
Build #1: This build is a bit more tanky then the 2nd build. For a starting item i would recommend Doran's Rings so you can sustain a good amount of mana while in the lane. As well as some nice harass landing those Sapling Tosses. But if you notice that they are hardcore attack damage in the lane that you are in you, then you might want to pick up a Doran's Shield. Also if i was to pick up a Doran's Shield, the chances i pick up Teleport would rise because you might not be able to farm as well with a Shield, so going back to spawn quickly to help yourself get back to your lane with new items and not missing out on xp would probably be good. I would say get a Banshee's quickly for the mana. If they have a lot of AP damage it wouldn't be a bad idea, but if they have a lot of AD you might want to grab an Aegis and then a Heart of Gold. Also of course you might not need all the items i listed every game, different opponents call for different items.
Build #2: Pretty much the same thing as the first build. The only thing different is getting a RoA for the AP and mana. It would give you more HP regen then the first one with the mastery Strength of Spirit. Also notice how in the first build i do not get the mastery Ardor, because it is not needed sense the only AP item you MIGHT get is Doran's Ring.
Seals: Mana regen per level for late game because of his ult, in fights he can soak up a bit of mana.
Glyphs: Early game MR because he is a bit squishy for a tank but that is because he's a bit supporty, so the early game MR will help with surviving.
Marks: Honestly for marks, if you want to get Armor pen or Magic pen that's ok. Because he is a terrible farmer before his E hits its 3rd level up. So last hitting with some Armor pen would help. But Magic pen would be good too for harassment and a bit more dps. As well as sense all of his abilities are Magic.
Quints: Early game HP for that survivability. But you would toy with this a bit and choose what you think fits best.
Seals: Same as the first build.
Glyphs: For this i would take late game MR, reason being is because with a bit more AP in the beginning you probably wont be up close and personal as much. So that MR can wait til late game.
Marks: Most likely want Magic Pen sense you have more AP than the first build.
Quints: I would get early game HP but you could probably get whatever you feel is best.
Use your E on the caster minions at level 5 will kill them if you have the solo lane. But if your in a lane with a team mate then at level 5 they are at 1 auto attack away from dead. Also late game his E is pretty good at farming. But also mid-late game his ult does pretty good damage to the minions, but ONLY use it if you know that you wont have a team fight coming up soon. (Buy wards.)
If you want me to go more into detail about anything else, ask in the comments and i will update this build. Thanks for reading.