use Q when ever someone is low healthed and use (totem) and Ult for low healthed people in team fights.
use Shrink Ray for running away or to cooldown someone like illidan with his burst so your team can attack him/her.

Ps: get so many globes as u can so u dont have to move back and forward for an healing fountain or so u dont have to do heartstone

(I'm not good as playing tank but i will try to explane the build)

As tank u always need to defend you're group and specially the healer!!
u wont get much damage because of Block, Avatar, Spell (D) and hardend shield.
And ofcourse use Avatar and hardend shield on team fights and use Spell (D) when you're low healthed and your without an healer.

when you have cooldowns just basic attack the minions or heroes so the cooldowns wil reduce.
Use tunderclap for little heals for little low healthed moments

Ps: this is not and Dps/tanky build this is full TANK!!

Use (D) to spawn creep everywhere use it all the time when your doing solo lane so u will get 200% health regen.

use rapid incubation when u are low healthed and have nearly no mana. like use it every time when your mana is 25% and or use it when your health is 50%

Use Ult so you're team can wait to kill the one in your Ult and or use it when your opponent has low health or use it when you're opponent is running away (use Ult on low healthed people on level 20 so when they die in there the cooldown will me reduced)

use mutalisks and you're infescted drop so u can push forts without taking damage or use them when u need to kill someone real quick (look out you're spells are very expensive with mana so use them good)

zeratul is an special character (nova aswell) cause those 2 can stealth and your opponment cannot see them.

use (Q) when you need to kill some minions (not allways cause zeratul needs to kill heroes) use (W) first on an enemy hero (2 times when your level 16) and then use (Q) so u will do an amount of damage to enemy heroes.

when u have 5 kills when you have qathering power u get 15% ability power so more damage when using an ability.

Use (E) to escape from an fight when you have low health.
Use (E) again to teleport back in the fight (when u think u can kill him/her (needs to be casted in 3 seconds otherwise it wont work)

The ability Void Prison can be used on different things
1. Use can use (R) to freeze your opponment(s)
2. Use (R) on you're core when your enemy is attacking you're core so they cannot attack your core so you have more time to kill your enemy.
3. Use (R) on a friendly hero with low health so u can kill the one that wants to kill the friendly hero.

Use (Q) to kill someone on low health.
Use (W) when u did (E) on minions or on heroes because they're vulnerable for 2 second.
Use ult on an group of enemy heroes and use blizzard when u did (E)

Use your ability (Bolt of the storm) to teleport away when u have low health
Use Ice Block when u have 50% health (when u have bolt of the storm not on cooldown) use bolt of the storm to teleport away before they kill u when your ice block is done.

say them in the comments below plz :P

Ps: if u wanna play with me check my profile plz!
Thanks