i've been rocking with morde lately and he's my number 1 champ. here's my stuff:
start tome and pot ==> hextech revolver ASAP (boots not as important in S3)
afterwards, if you can, build needlessly large rod (i know some people like to build tanky first, but i've found he's much more effective with AP)
if you're having trouble with the AP mid, start byuilding an abyssal scepter, else; get haunting guise/finish rabadons (in some order), and get kage's lucky pick (i usually turn this into WotA later on)
finish liandry's torment (sorcerors boots should have been done a while ago now)
a good final build, for me, is sorcerors shoes, rabadons, WotA, Liandry's, Ryailai's, and some defensive item (spirit visage, warmogs, zhonya's, etc; i usually never get to the 6th item)
other items to consider are abyssal scepter (VERY good item, especially if the other team is AP heavy), and Gunblade (if you're finding yourself very much ahead). i haven't really tried deathfire grasp yet, since i'm not that good with activated items, but i can see how it could be devastating if you find yourself facing a ton of high HP bruisers; peole also swear by Warmogs, but remember; it is ALWAYS better to build AP than health on Morde
before level 6, you are extremely vulnerable to ganks, so be aware of which bush the jungler comes out from, and save your flash. once you hit 6, when you are ganked, ult the squishiest target first, then w-q-e-ignite. if you can make it under your turret, this should be a double kill. This is much more difficult if the jungle is, say, cho, since his high hp is difficult to handle until you get liandry's.
at 6, you should make your way to bot lane. if you have AP, you should be able to help your team get at least 1 kill.
if you have any more questions, just ask!