The most important thing with Malzahar is farming (You should take the middle lane!!!). At lvl 1 skill "Call of the Void" and use it 3 times at the spawn, so your passive little creep will come when u use your next ability. Wait for the first enemy minion to be below 100 hp to get your first minion kill. Now your creep will spawn and get 3-4 further minions of the first wave.
Until level 6 just dont try to kill your enemy champion. Focus on the Minions, so your enemy isnt scared of u yet and keeps a close distance. As soon as your lvl 6, use your flash to get close to your enemy and get your first Champion kill. IMPORTANT: ALWAYS use Malefic Visions on your enemy before you cast your ulti!!!
On lvl 7/8 u can start ganging on the top and botton lane! Use Call of the Void to silence your enemies before u rush in. Call "Null Zone" on stuned/slowed champions and spam your Malefic Visions which jumps from enemy to enemy as they die in time, to kill champions who run away.
If your enemy team has DPS champions make sure u focus them! Use "Call of the Void" to silence as many enemies as u can. If a Enemy like Master Yi jumps on to you, cast Null Zone on yourself, Malefic Vision on him and after that cast your ulti on him, so he cant do his damage and lifeleech.

Pros:
Able to Silence
Null Zone very effective on tanks
Malefic Vision jumps to the next enemy, if the first died in time (4 seconds)
Ultimate makes enemy unable to cast/run


Cons:
Silence/Stun stops you from casting your Ultimate
Low armor and magic resistance

Use Call of the Void to silence the enemy and Null Zone on tanks.
Spam Malefic Visions as often as u can either on minions or champions
Make sure you cast your ultimate on those enemies who are dealing the most damage, so they are unable to cast/move and die.
Stay behind your teammates, because you are the one who gets focused because of your massive damage.

I take "exhaust" and "flash".
"Flash" to get out of focus or jump to the enemy to get your spells casted on him.
"Exhaust", because Malzahar has low armor and magic resistance so if a enemy champion wants to nuke you, you can cast it on him, to redruce his damage. For example if a Master Yi runs to you and u have cooldown on your spells, you can stop him from killing you, before you are able to cast your spells.

If your dont like "exhaust" you can also take "ignite", which is very nice, because u can cast it while u use your ultimate.

It's very easy to farm with Malzahar, because Malefic Visions jumps from enemy to enemy, when they die in the next 4 seconds after casting it. Make sure u also attack the enemy minion cursed by Malefic Vision. Everytime a enemy cursed with Malefic Vision dies, you get mana back, thats why u can cast your other spells very often without loosing too many mana.( Try to damage the enemy Champions with Null Zone and Call of the Void, while Malefic Vision kills the minions.

After 4 casted Spells you summon a little creep which isnt controllable, but deals great damage. It will help you to get minion kills, thats why u skill Call of the Void on level 1, because your Malefic Vision only deals 43 damage on level 1.