This is a pointer in the right direction on how to play this champion while the items are important the how you play is most important.
The items to pay special attention to is the way you buy the first three items. Your going to want to start with boots and health potions this puts you in a good position, along with flash, to not get caught and killed early game. The next item after boots should be catalyst, this is so that after you go back for catalyst that you don't need to go back anymore after and can stay in your lane longer due to its unique level-up properties.
After getting the catalist (which should be around level 4-5 possibly 6 if your not last hitting well) you will want to stay in your lane as long as possible pushing it fast and then going to lanes where your enemies are past the halfway point. In most cases ulting the enemy at the lowest HP in a lane you are ganking results in a kill by you or allows the people in the lane to get the kill. Your goal when you go back to base next is to get the Needlessly Large Rod and to Upgrade your boots. Either one alone is fine. After your Deathcap and Rod of Ages you can go in all different directions, a sample order for the next 3 items is shown to include HP and CDR. Alternatively other items such as banshees veil, soulstealer, moonfire spellblade and hourglass are all great situational choices.
The skill sequence for Malzahar is very strait forward you want to level up your dot as a priority for getting the bonus gold of last hitting minions. While it isnt required to put the point in null zone before level 6 it is nice for using Malzahar's passive to its fullest and for killing someone 1v1 if you don't have the option to gank other lanes.
Summary and Overall Strategy
Early game is detailed in the Item section and as for late game: It is of great importance when you use you ult it can catch someone out front and give your team the quick 4v5 teamfight or lock down the enemies highest dps from a nice range away.
The most success I've had with it isn't by myself when i need to cast 3 things before ulting but when i can just ult someone and have at least one teammate with me to take advantage of the 2.5 second suppress. It is also important to note as a general strategy to use the dot (Malefic Visions) on people as a harass move, don't just go all out with every move at once every time. At the end of the game constantly getting your dot on enemy champions ticks them down slowly but surely and provides a really decent harass.
It is also important to note as a strategy to block skill shots it is nice to have your Voidling ready to come out on the next spellcast! (If you dont know what that is look at his passive) An example of why this is important was demonstrated by my brother today who had a rammus coming at him full speed in ball curl and he casted a dot on him and his Voidling popped up stopping the rammus gank in its tracks. it can also be very helpful to have a Voidling up and running behind you to block skillshots shot from behind (Rumble, Ezreal, etc)
The ideal runes are the ones I have displayed but slight variation is always fine.