Well i just bought him and i really dont see him like a much a big deal but that's prob cuz i'm playing him wrong, anyway, i mostly farm with Q cuz his AA are really weak and have low rage so i dont want to get poked to death, doing this i dont have much farm (70-90 at 15 or something) i'm guessing with some practice i can do better, but my problem is how to build him, since he really have mana problems when you use your E to kill and since he needs **** loads of dmg for his ult to be usefull, I dont know what to build first! plis help me so i can ULT PENTAKILL! (lol jk)

try learning Jungle Karthus
http://www.youtube.com/watch?v=ViEATh0M7ZU

Its going to be hard, but very rewarding, you'll have a lot of farm(more then mid) and you dont have to deal with harresing issues or lane issues, so if i were you learn the secret ways of jungle karthus

Always rush Catalyst; it will help you sustain somewhat in lane and give you a good chunk of health. Rush RoA as fast as possible. Next, go for Deathcap for damage. After that, it's situational.

My typical Karthus build looks like:
Sorc Shoes
Rod of Ages
Deathcap
Void Staff or Abyssal (I lean more towards Abyssal)
Ryali's Crystal Scepter
Hourglass

this isnt a bad karthus build but its pretty much the standard build for any ap caster.

for karthus who needs lots of mana and health to stay useful there are more concntrated items that do the trick too.

my build for karthus ends up as such


Sorc shoes
Rylai's crystal scepter
Athene's unholy grail
Rabodon's deathcap
Zhonya's hourglass
Moonflair spellblade or Wota

This guy, you want to follow his build, i do exaclty the same thing, but i go boots, rod of ages, void staff, death cap, ryalis, hourglass.

Karthus is actaully an amazing farmer, you just need to practice more with his AA. Just keep in mind that his Q does more damage when it hits a single target.

So to max out karthus in his abitlity to lane, i HIGHLY suggest getting -

3 Flat AP quins. 9 AP per lvl Glpyhs. 9 Armor Seals. 9 Flat AP Marks.

Masteries - 22-8-0

With that, youll have some durability and increase your damage output.

Also, remember that a Mana Karthus build, is also viable, being able to spam your Q and your E in the middle of a team fight, can cause massive damage.