Tips to Use Witch Doctor (Dota2)

• Try to wait on casting Death Ward in teamfights until the enemies have already used some of their stuns, making it less likely you will be interrupted.

• Casting Death Ward from a hidden position such as the trees, or uphill in the fog, makes it much harder to disrupt.

• At rank 1, Voodoo Restoration is the most efficient form of AoE healing in the game based on mana-spent-per-hitpoints-gained. It is often a good idea to invest a single point into this skill during early laning and then max it later after your other abilities once you have the mana regen to support the higher cost.

• Paralyzing Cask is most effective when there are just two enemies close together. Under ideal conditions you can stun two enemies for up to 4 seconds each with this skill.

• Casting Maledict on opponents will often immediately force them to retreat for fear of taking crippling damage while under its effects. You can use this to make enemies flee who would otherwise easily kill you or your allies. If they do not flee, then at best they're trading themselves for you. 

source: http://dota2.gamepedia.com/Witch_Doctor