I am just level 12 and I have been playing karthus a lot. I have only 9 wins and like 14 losses, I have never gotten higher than a 2 win streak with him and usually get like 3 losing streaks. I have tried everything to get good with him, all the strategies. My friend says that if your not good with a champion at first, you will probably never get good with them. Is this true? Also I like playing ap mid, but I keep getting this with evey ap mid character, I do even worse with other roles. I know how to play karthus the most but this is my problem, what can i do?

Play passive at first and slowly learn to be more aggressive. It's a lot tougher to "make a play" by going aggressive than it is to just passively farm your lane until laning phase is over.

Focus on getting as many minion kills as you can during lane phase. If you're 0/0 mid as Karthus by the time the first turret goes down, and you have more minions kills then your lane enemy, and you didn't let mid roam around to pick up kills, then you can effectively say that at least during lane phrase , you didn't screw over your team.

Also , just a general Karthus tip - use your ult when a fight is going on, not when you think you're going to be able to last hit a kill.

I doubt you know how to play karthus properly at level 12. Farming is incredibly important on him. Pre- 6 you should be only be farming and harassing situationally. Try to catch minions by themselves with your Q so CSing is much easier. Build him correctly.

Generally with Karthus, you should just dive into a team fight with your AOE, die, spam Qs and then ult, however, this purely depends on how you build him.

If you can farm properly at summoner level 12, ( 80-100 cs at 10 mins) you should dominate the game, carrying your team every single game. Obviously this is not always the case at higher levels as people begin learning how to counter karthus.

Have fun.

What would ever cause you to believe someone who says this, in any context? Of course that's not true.

But I DO those things, I only farm, but there is always someone from top ganking me on like 3 seconds

Wards

Try not to push your lane so much. Karthus' biggest advantage in lane pre-6 is his range, so as long as you aren't pushed out you can stand pretty much under your tower and still get last hits. And if you do get caught out your Wall helps a lot with running away.

After 6, let your team know you have your ult and then keep an eye on the minimap. If you see a fight (and actually look, don't just rely on the map) that it looks like you can help with, ult. ~250 damage can completely change the course of a fight in the early-mid game. Also ult immediately (as soon as you're safe) if an ally calls for it.

As a Karthus main player, only use your Q to hit a single minion for last hit purpose (which is, you have to put it very well if you want to hit the middle caster minion), in this way, you will never push the lane and never get out of mana
Always tell your team when you hit 6
Lastly, wards are important

Your friend is wrong, especially in relation to new players of the game.

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.

karthus is not that great to be honest. just pick diana, goddess of the moon. or katarina. who has the famous occult infinity sign on her daggers.