[BUILD EDIT] In the 1st build you've to choose between Zhonya or GA, in the 2nd one you've to change your Haunting Guise with a Void Staff as a last stuff.

Malzahar is a powerful wizard, he deals pretty hard damages from the beggining to the end of a game.

In early game :

Malzahar is wasting a lot of mana when he uses his spells, that's why you need a tear of the Goddess as soon as possible on the game.

To harass and farm : use your E on minions who are low end push it to the other by auto-attacking them, try to take the poison on your opponent without being touched by him, it's better to wait poison lv 3 to begin to really bully your opponent. Don't forget that each time your poison is going on another target, he gives you a bit of mana, it's really important to play with this to not be out of mana after 3 spells.
Your Q is easy to cast, and apply silence on the targets touched by it. If i can give vous an advice, try to put it something like the distance done in 2sec if your target is moving, if they back pull it before them if they come do it too, if your target is immobile, put it right on her.
Your W is used to help the pushing and to permit your poison to kill minions and go on another easier, it xan also be use during fight, especially team fight where he can deal tons of damage.

In Mid game (lv 6) :

Your ulti is UP, you've to look at ennemies build to know if someone has a quickslash which could counter your ultimate, be aware of ennemies builds is really important !
Try to harass your opponents with your poison, while they are 2/3HP, use in a few sec E then A then R (you can also put the W before your R but it's kind of hot to temporize). From their, your opponent will die, if he stayed alive with a few HP try to reput him your poison fast before going back to safety, if he use flash don't follow him, it's probably a trap after him.
Don't hesitate to go through the river or the jungle to gank other lane, you're powerful as hell don't forget it !

In late game :

It's hard to play Malzahar in late game because of his monotargeting, but if you're a great player you can even deal tons of damage : use your Q on the way the ennemy team is using to silence and deal high damage to them, use your W on the melee to deal high damage based on targets maximum health and don't forget that your W is even good to kill Baron or the Drake.