Your main playstyle is going for a pull with E then using an autoattck into Q procing your passive and then chasing down your opponent with the bonus movespeed from your passive.
If you miss your E sit back and farm with Q until it's up again. You wanna look for all ins over and over.
When you have your ultimate if you land an E use ult immediately as they will be close to you already putting them in the ideal spot for you to use your ultimate.

Mordekaiser's Early game is pretty awful to be honest and all of his spells have per level scaling making him better in mid/late game which is where he shines as a champion.

The best thing for you to do early is farm as much as you can without dying or taking to much harass and trying to set up ganks for your Jungler.
Once you get level 6 you're going to be in a better spot to start looking for fights.

Once you get 1-2 Items and have ultimate you wanna look for all ins.

Depending on your lane opponent you can use your protobelt once you have it to clear your wave as fast as possible and run to a different lane for a gank. But if you roam make sure you're not gone long enough for your enemy laner to get more damage on your tower or get a big CS lead.

sometimes it's not ideal to roam as tower plates give you more gold than assists.
Let's say you're vs someone like Talon it might be better to just let him roam while warning your team that he's coming and to be careful so you can get tower plates and pray that they listen to your pings and don't die to his gank, which is going to give you more gold and more lane pressure.

Mordekaiser will die extremely fast if he get caught the only survivability he has is his W which doesn't last to long and is only good once he has built up some shield from fighting.

So as Morde you want to look to catch someone out of position with your ultimate and kill them without getting yourself killed right after.
Or another option would be to ult the main threat on the enemy team out of the fight allowing your team to fight with ease even if you die you gave your team the chance fight without problems.