Feel Free To Provide more info or correct something that i may have stated incorrectly.
Remember Most of this info provided is from personal experience through gameplay.

Try to farm up and scale into mid game and late game. His early game isnt all that great simply because he gets harrased and poked alot in lane. And at this point in the game he does not simply have a way to stick to targets without a supports help. But once morde gets going he becomes a killing machine that can blowup anyone.
Just remember be wary of getting kited. Other adc's can kill you even if you do one shot them, you still have to get to them. Tristana and lucian are very scary to go against. You will probably cry or brake your keyboard trying to kill them without flash or a team to help.

Seals: scaling hp runes or flat ad
Glyphs: Magic resists x9 or 6 flat magic resist and 3 flat cdr
Marks: Hybrid ap/ad, even though ur mainly ap you still have alot of ad so i reccomend hybrid over anything else.
Quints: Either flat ap or you can go ad or attack speed depending on your playstyle :D

Cunning Skill Tree
Wanderer: Overall the better choice in the cunning skill tree
Assasin: Can be swapped out with affinity or stash. All preference.
Merciless: um well if case you didnt know u dont have a mana bar :D
Bandit: I take bandit simply because its really nice to get extra gold for free. And its not that i dont take dangerous game every now and then. its mainly because morde scales really well so i want to hit those power spike items as soon as possible. But Dangerous game is just as good if not better. I just pref the gold remember your choice.
Precision: Just overall better because it scales and you do mixed damage so its really nice to have.
Thunderlord's Decree Vs. StormRaider's Surge:
Both very strong on mordekaiser. One is Extra Burst and the other allows you to stick to a target or even escape if things are looking bad. The movement speed is SUPER NICE! But i like decree just because its so strong atm. Later on when it gets nerfed i will prob always take surge. if it gets a nerf

Always take flash just because the utility of flash is so nice no matter who you are playing. Besides you already have no escape or engage, no need to ask for the opposing team jungler to camp your lane.
I take heal bot lane simply because its a good bait.
Other summoner spells you can use to replace Heal are listed below
Ignite: Lane pressure, not as strong if your support takes ignite as well, and your alot easier to burst since your really squashy early on.
Exhaust: Is strong on morde simply because its a slow and makes ur opponent do less damage and take more.

Try to Farm as much as possible to get those power spikes morde loves so much.
Take jungle camps when you can.
Good times to take jungle camps are when you shove a wave into the enemy turret using e.
Or if they are freezing the lane against you. (Dont sit there and let them waste your time, if you go missing they will start to shove and you should hopefully be there to soak up all that farm and exp.

PROS: CONS:
High Burst Gets kited really easy
Strong against tanks (cant break passive) Melee in bot lane
Can one shot carries and make them yours to do Squashy early
as you please ;) Can be zoned hard
CAN OWN A PET DRAGON No escapes
Strong at taking objectives with dragon Easy to get ganked
Simple Kit

Overall Mordekaiser has his up and downs, but fortunately he has the carry potential to win games on his own.
Make sure you stay farming, ask jungler for golems or gromp if you are getting zoned, so when the enemy starts to shove you can have your jungler gank for you if possible.
Try to secure dragon because that usually means you can go mid and take turret or go baron and have dragon tank. Dont waste dragon bot lane unless your taking turret. Better to group with dragon as morde provides alot more pressure.
If you have any comments/concerns please feel free to let me know, If anything i entered is not accurate please feel free to comment. Criticism is welcomed :D