Well, this has been working for me.

The first item you should start off with is a Regrowth Pendant and a Red Soda. These items are useful in the beginning because, you know, it regains health for you. (Morde Skill's uses his health duh.)

The next item you should rush is the sorcerer's boots, this it to do more damage.

(My friend always ask me why I do so much damage but I have no damage items? Well, I reply, MORDE ES #1!)

The next item, depending who your enemy team is playing, should be Randuin's Omen or Force of Nature, also known as FoN.
Randuin's Omen = Against AD (Obviously)
FoN = Against AP ( Duh )

I usually start with Randuin's Omen because people like to play AD.
You would want to buy Warden's Mail because it gives you stats against Ad already giving you a better chance to chase them down and run away.

Next get the Heart of Gold, it give you money to finish it off.

Yea, I know, why not Hextec build? So far, every time I would play against a morde with a Hextec build, I would end up killing him and using his ghost to beat on other people down with his mace. Two Mordes, ah, scary.

Well, then, my idea of playing morde is simple, tank but for some reason hurt, that is why you should rush Abyssal Scepter, this is for the little AP champs the enemy has and also destroy away their magic resistance giving a chance to hit them big.

Just like the last item, rush a void staff, to do even more damage! Also, kill off the rest of their magic resistance.

Since you have all that down, the last two items can depend on how you are doing, if you are taking more damage then you want, you can buy more tanky items, I prefer Guardian Angel. Why I prefer this is because it doesn't matter how much life you have, if you can survive, you win.

Ok, here I go off with a story...
Once upon a time, there was a boy name Mordekaiser, he was getting picked on by a girl named Akali and her friends, Vladimir and Rumble. While they were hiding at their base, Morde ran up there to destroy their turrent, and as he did, Akali and her friends ran off to chase him down, and killed him, but little did they know, Morde had Guardian Angel armor, he revive himself with 750 hp, then tried to run to the bush as he got attacked even more. But then, when they chase him to bush, he turned around, and attacked all three of the bullies. Til this day, rumor has it that he never lived, but those rumors just lies because Akali and her friends never lived.

Ha ha, I know corny, but true, they were saying they had 100% chance in winning because Akali was fed, ha ha, but you know, we ended up wining, and my kills ended up high ha ha.

Well yea, get Guardian Angel if you are hurting a lot and want to stay longer on the field, get the deathcap if you don't die that much and you want to do more damage, or get anything else that would help you out in the late game.

PS: Try ganking, if you are laning with someone, make sure that someone can solo lane when you gank or jungle a bit, the other team will get scared.

Also, if you are playing against a trynd, when he ults, ult on him. Trust me, he'll die.
You also want to ult on the carry Dps when playing him, ult him and ignite at the same time.
Harass with "E", use "W" on yourself when farming or in lane, but when you do it, run up to the minions, the other team will back off. Wait a second or two after using "E" to use "Q" when Laning in the beginning to have enough time to use "E" again if the other team comes up.

And one last thing.

MORDE ES #1