LvL 30 Summoner new to Karthus need help

[quote=Neublet]Ok so I just hit level 30. I am used to playing Tryn, Trist, and Alistar. I have been saving forever to get karthus and Ive finally bought him. I have checked leaguecraft and mobafire to find good builds, I've even read some here. I would like the advice of a level 30 summoner who uses karthus in ranked games and is fairly decent. Im working on getting my coordination down im finding it hard to hit with lay waste but im learning. i have had a few issues with his ult trying to kill morde when he had next to no health but i think thats just going to be something i learn in time. Is karthus good for ranked games probably in solo beings I dont know really anyone whos 30. Should I consider maybe Fiddlesticks? or should I just stick with one of the 3 I know best? If you have a link to a guide please share it, or please just tell me what you use. I dont want to be a failed ranked player in solo or on a team..im willing to learn. thanks![/quote]

[quote=B-Wong]Erm... It's practically impossible to kill Morde when he's at little -> no health with your ult, he'll just Siphon of Destruction then he's set so don't worry about that too much. Just wait until enemies are low then ult. It's pretty easy. Don't do it in the middle of team fights, wait till everything has settled down THEN ult. Don't use it to try and KS. Always get a Mejai's. Tell your teammates to tell when someone is low. Basically Low = dead with Karthus then after you feed off of lows with your ult carry.[/quote]