One thing you don't need with Morde is HP. That's the big mistake tha most guides do. Since u got your shield, you have to get more AP and Resistances, cause your shield gives you enough HP to "tank" or w/e. Rylais sometimes is a good item though for it's passive,but not in early game, early is much better with damage. Also Deathfire Grasp used to be an unbelievably good item for Morde, combining with your ult,you could one shot the carry in late game,just by dfg and ult. Hextec, Abyssal, Zhonya's and Rabadon's, are "must" items. The last item, depends on the game, and u can do void staff, dfg, or rylais, thats up to you. Also the item sequence you follow is depends on the game you are playing.

One standared way of farming is taking your lane twice,and the wraiths once, cause that's the time that wraiths need to respawn. Also you could go "lane-wraiths-lane-wolves" and goes on like this, but that's when your jungler does much counter jungling or ganks a lot,cause with that way you get much of his farm. Also you go "lane-your wraiths-lane-enemy wraiths" but for this you need good warding so you don't die in their Jungle.