Maokai. He's wood. And hell, he does a damn wood job at being wood if you ask me. So:
This guide aims for:
Inspiration, mainly. It's no babysitter.

Wood about wood:
- Badass
- Shiny eyes, cute Saprolings
- strong at changing the flow of a teamfight with his R, Q and W
- decent tank, great offtank

Bad about wood:
- slow movement
- team dependant
- with this build his earlygame leaves him comparably vulnerable

I leave this section open, because it's fairly optional. Personally I use Flat-Health Quints, M-Pen Reds, Blue CDR and Yellow M-Reg and it works fine for me.

Two choices here:
Either 9/0/21 caster or the strange one given above.

Start with a mana crystal and two pots (red, blue), so you are able to harass better.
First visit at your fountain should be when you can get Catalyst and +1 Boots.
Later you upgrade to Banshee's Veil & +2 Boots of your choice. Usually that'd be Mercs, but Sorc. and Ionian's make for a good option too.
Heart Of Gold to Banduin's, Glacial Shroud to Frozen Heart.
That's pretty much it. Later on you might get Rylai's and a Guardian Angel, but if you think you know a better option that is more suited for taking on your opponents feel free to get that one.

Start with Saprolings, they're cute and useful.
Able to annoy the **** out of your opponents with their range and also able to facecheck bushes for you. Make sure to get one level in Q,W,E until level 3.
Im not going to give you a certain sequence for later on. You should do fine if you just level your ult when you can and spent the rest of your points rather even.

Flash/Ghost.

Slow wood is meeeeeeeh. Flash and Ghost are great for catching up to that ****ing Irelia thats beating the **** out of your little carry-friend your maintank can't save cause he's caught between two aoe stuns.
Nice to initate with your root, shockwave, ult, saproling combo and make it out alive: Flash.

Go for duo-bot. No mid, no solo, no jungle, no ****. Ranged characters are a good option, so you can help them with some early kills. Taric and Soraka are surprisingly good too.
Early game your harrassment abilities are quite cool. Not much on the bursty side, but your cooldowns are pretty low. If you're with a ranged champion, your root can work wonders and when there's one of these bursty melee guys with you... shove that squishies towards their hungry purses.
Mid and late game you're playstyle should be more tanky. If you're the only tank on your team ffs protect your own team before you try to chase down that Singed to their nexus.
When there's another tank, e.g. s1 like Rammus or Shen, you can go for a more aggressive approach and try to disrupt your opponents carries, mages..
Use your ult when the first one or two enemy ultimates are used (timing needed. and brain) or when you're sure their not getting away from your mates.
Yea. That's pretty much it.

Woody: Fun to play, resourceful piece of wood ol' wood.