Your friend is wrong, especially in relation to new players of the game.
Originally Posted by WLANWPS
But I DO those things, I only farm, but there is always someone from top ganking me on like 3 seconds
You should only be last hitting, don't push your lane. Also bring at least one ward back to lane every time you were at shop. With Karthus especially, he's a champion that lacks escapes and is vulnerable to ganks. Farm at whatever pace you can, it's okay if the lane pushes to your side, every Karthus player has to learn to farm under turret.
One thing I think about playing Karthus at new/low levels is that it might be a bad idea simply because Karthus is a passive farm lane, and he shines in the late game. At low levels people feed a lot and they generally don't know how to team fight or even make it to late game, so it may be better to pick Karthus up again in the future.