I would recommend checking out his Champion.gg page for Mid lane as a good start.
Now as a Malz main myself https://i.gyazo.com/64f63a76371f3e87ebed23cebf88fc51.png and still loving him since the rework https://i.gyazo.com/2fefd331fdbd069a754f291ed60d3da0.png .
I will give you some advice that I have learned from my own playing as him and experimenting with him.
First off: Start with Malefic Visions, this will give you a good way to keep your mana up for a LONG time if you are able to control the minion wave and get it to bounce often. Get Void Swarm second because this will help you push that wave under the other laners tower and make CSing almost a living hell for them. This is where his Champion.gg mid lane and I somewhat disagree, but I can see why putting another point in Malefic Visions is good. I like getting Call of the Void 3rd since I can use it to refresh my Malefic Visions if it bounces onto my opponent and make it last as long as it possibly can.
Me personally these are some items I really enjoy on him:
(Morello's and Rylai's are already really good on him and essentially core on him) I will go into detail if you would like as to why I include Cleaver and Gunblade. I hope this is a helpful good start.