What you want to have is a jungler in your team so you can get solo lane against 2.
If not, no problem because morde also go's good in having a partner sidelaning.
As mordekaiser all you have to do is stand behind minions, charge up your shield with Siphon + Creeping death. Once your shield is over 50-60% you walk forward and keep your shield up by hitting enemy players with Siphon.

I have builded a itembuild of how i usually buy the things, BUT !! be aware, that if u have a full AD team a force of nature will not help at all.
Look at your lane enemys, if one is AD and the other AP, see what hero does more harass and damage, and build your armor / magic resist towards the item the enemy players are building ( AP -> Magic Resist / AD -> Armor ).

Once you reach level 6, wich is most likely before the enemy team wich you are laning with are, since you are a very strong pusher.. Have your ignite ready, you are gonna need it.
It's best for your lane partner to harass the enemy champions to about 50% health.
Then i normally go down in HP to bait them, since not everybody knows what mordekaiser can do.
You use your ultimate + ignite instant on him, walking towards him, Siphoning him with your Creeping Death on yourself, trying to hit your Mace of Spades on him. As soon as the target dies Alt click on the other enemy player alive.
You will want a Ranged as your pet, since it does great damage.
Mordekaiser is one of the best mid game heroes i know when it comes to ganking.
Farming minions will be awesome with Siphon of Destruction. If a round go's well you can also consider buying a sunfire cape, it stacks with your Creeping death nicely, and it will give you 50% more chance of lasthitting a minion, gaining more gold.

Once you reach level 11, try to have ignite off cooldown again, since your children of the grave will do marvelous damage in combination with rank 11 ignite, abuse it.
Depending on how the round go's and depending on what your enemy team is building, you counter build magic resistance / armor.
At higher level teamfights will kick in.
If you are the ***isgned person to engage, be sure there is a enemy minion wave, to charge up your shield. Tell your team to go for a specific ranged enemy, make him your pet and you have a huge advantage of having 6 players against 4.
When you have full set, i hear people say: ditch sorcerer's shoes for a AP item or more armor. This is ********. I tried that out, getting killed constantly cause i couldnt run away anymore. What you can do, if you see the enemy team having almost no magic resistance: Buy Boots of Ludicity. Cooldown reduction is very effective, since your shield will be up even more.

I have finetuned this build the way it is, there are no AP items in my build because this is a tank guide. In my opinion: Tanking with morde is not just taking damage. Its basically a battletank. Once you have your pet in teamfights gear doesnt matter anymore, but your pet can do all the damage for you, while you are not dying easily. With AP morde, you do awesome damage, but then again at what price: Being squishy -> not having the ability to use your pet as long as you would have with tanking gear. The team will see you go AP and focus you in teamfights.