Maokai is a ap tank that can fill many roles in a team. He has a great initiate ability, hes good at chasing, and even has a built in ward. however its important to note that because he has so many roles on a team, its hard to do all of them and his abilities might seem "watered down" as he don't excel at all of them.

Pros
-good initiator
-good early game harass
-sustains in lane well with passive
-ult is amazing in teamfights
-built in wards

Cons
-mana starved early game
-no really good getting away abilities
-saplings are ******ed some times

I like to start off with a meki pendant and mana potions. The reason being that you can regain hp easily with your passive. I then build the meki pendant into a tear of the goddess and get boots. Now i start working towards archangel's which by its self fixes mana problems for almost the rest of the game. After that i finish my boots usually picking up mercs but depending on how much cc they have sorcerers are also fine. It's also okay to get rod of ages before archangel's but still getting tear of the goddess first. Then i start building tank items usually sunfire cape (for health and armor), banshees (fon is also okay), and i finish it up with either a warmogs or guardian angle. but if you think you have enough hp dont get warmogs.

Clarity is a really god summoner on maokai because it allows you get some early harass and help your teammate lane. Teleport is also good because maokai is such a potent ganker and allows you to get to lane faster. Other summoner spell you can take are flash (helps get out of tough situations), ignite (somebody gets away from you with 36 hp? cant chase far because your un a team fight? lets fix that)

Moakai is a saver of *** he has a root a slow/knockup and an ult that reduces damage what more could you want in a laning partner? you want to snare enemies who are running away so your teammates can catch up, divert their attention from your carry to you, and bravely "q" enemies that are chasing your team risking your life but its okay because their shower of gratitude is well worth it (sorta).

Maokai is great in lane hes pretty much ungankable if you have your saplings in the the right places. He can also lane for a long time restoring health with his passive. but what you wanna do in lane is harass other champs with your saplings and punish those who over extend your your root.

Maokai is a great farmer, his saplings can easily take out a wave of minions if placed well. your q is also a viable option.

initiate with your root, then knock them up with your q. i believe you should wait for the full root duration as im pretty sure your q knocks them out of it 9correct me if im wrong) some times you want to wait for a teamfight to begin before you use your ultimate instead of using is right after your basic combo w then q. what you can also do is pick off dieing champs with your saplings as they hide in the back of the fight

Overall maokai is an amazing champion and is highly underrated. he can have some mana issues but archangels will fix that, and during mid to late game you can leave it on for a very long time.