On the early game, focus on farm, you need to be in level 6 the faster you can. When you gain your Requiem, you will be able to help your teammates with it, and steal some kills too, so be atention on all the game. When the Rod of Ages on your hand, your poke will be much more powerful, so use it to advance on your lane and help another lanes, if they need. When you finish your Rabadon's deathcap, you can start your "suicide style". Throw yourself on the middle of the enemies, and poke then the most you can, when you die, see if you can use your Requiem to kill someone, if the answer is "no", only poke the better you can. If some enemy champion flee from the battle with a few points of HP (it normally happen), revive and use your Requiem to kill then. On the late game, if almost 800 of ap damage, these style of playing with Karthus will make your team win almost all of the teamfights, and you will be able to kill a lot of enemy champions, so, enjoy your Suicide Karthus!!