Here's to a nontoxic, well-meaning new player! Glad to see you're actually having fun!
There have been a lot of replies, so I'll try not to step on anyone's toes when I go in and give advice. I figure about the only thing that hasn't been covered is branching out your champion selection. A lot of new players stick very closely to one champion. I've seen people who are max summoner level that say that they can only play one champion. It's refreshing to see someone who actively looks for new champions to play!
Some champions make others easy to learn; if you've played a lot of one champion, there are others that are similar enough to that champion that they would be relatively easy to figure out, but are still different enough that they have advantages and disadvantages. A champion that comes to mind when thinking of Katarina is Zed. You play him in the mid lane, you have some abilities that function a little like Katarina's Shunpo (E), and her Sinister Steel (W) but has quite a lot more... mobility. He does have an energy meter, which is somewhere between katarina and a mana-based champion; energy is kinda like magic, but it caps out a lot lower. However, abilities typically have lower cost, and energy comes back much faster than mana. Anyway, Zed has a similar reputation as Katarina for being very strong, very difficult to beat, but not as difficult to master.
Another very mobile melee mid is Fizz. Fizz is also know for carrying the game hard and dealing really high damage. He also has one ability that makes him very slippery and hard to catch.
After you master some of these easier champions, you can get into more complicated mids like Yasuo. At the current moment, I don't recommend you take up Yasuo, because, yes, he has a lot of power. If played right, he may have just as much, if not more, power that Katarina. However, this power is locked behind his complicated mechanics. Many of his abilities are, as many community members would say, "overloaded." This means that most of his skills do multiple things, or behave differently under certain circumstances, making him versatile, but as a result, complicated.
So in a sort of gradient, those are some recommendations. However, if you would like some suggestions for other roles, I suppose I could do some of those...
Taking into consideration that you're comfortable with Katarina and Fiora, you might want to look into assassins in roles. For instance, you may want to pick up master yi for jungling when you get to learning how the jungle works (I apologize if you already know how to jungle). Yi tends to be more like Fiora. You build AD, you are pretty fragile, but when you get some kills, you're darn near unstoppable. What's more is Yi goes freely around the map, getting mostly free farm without people trying to poke him, and as far as the enemy knows, you could be anywhere when you decide to drop in and attempt to murder someone.
You may also like Pantheon as a top or jungler. He's pretty equally strong in both roles. He has a bit of a safer playstyle than Fiora, but he has some pretty awesome powers that could be neat to try.
As for adc's, you could probably give Caitlyn a try. She's known for being good for beginners, and has many tools that keep her strong in the later levels as well. She also has the longest base auto attack range in the game. Graves may also be a good choice. The adc role is probably one of the least diverse, because all of them have to be long ranged, auto-attack based, with some form of attack speed increase, or something that makes autoattacking worth while. That's not to say they're all the same, but they mostly have to have those attributes. However, there is one that I recommend you stay away from for now, and that would be Kalista... she's pretty complicated, and her ult is very risky, or could end up backfiring if you don't know how to use it properly. Even then it still is a bit of a gamble. If her kit really impresses you, then you should probably try Twitch first, as he has some slightly similar mechanics, but in a way that is much more user-friendly.
As for supports, this role may not be your cup of tea. Supports are rather passive by nature, perhaps sometimes passive-aggressive. As someone comfortable with assassins, you may want to lean towards more agressive supports like leona, morgana, blitzcrank, or thresh, in relative order of difficulty to play. Leona is probably one of the easiest, because she's difficult to kill, and has loads of cc, morgana requries a bit more of a skill shot to land her cc, but does pretty good damage. Blitzcrank has a high-risk high reward pull that can change the game if you land it reliably, and thresh.... oh, boy thresh. He has a similar skillshot with his hook. The difference is that he has a slight windup before he throws it. If it hits, you can drag people around a bit and jump to them before the break free to do some damage. His other skills make him one of the strongest supports, but they need to be used wisely. For instance, his lantern can be unruly, but you can save lots of people with it. When they click the lantern, it will teleport them to you, and it also grants a shield to allies. Flay is another one of his skills that's pretty unruly for beginners but is a godsend if you know how to use it. His ult is also one of the most powerful slows in the game, so he has a lot going for him if you know how to unlock his potential.
If you'd like to play a few games with me, I could be up for it. Just add me in game, and I could give you tips. I'm not the best player, but I have some good game knowledge that could be helpful. Not to mention, I am a pretty good support jungler (or a jungler that mainly secures kills for his teammates when he comes to your lane to lend a hand, as well as towers and other useful things) so it makes for a good time when giving people tips.
oh yeah, also, sorry for the wall of text... I just like helping people around here who are willing to listen and learn instead of just ranting ^/////^
and items that will help her when she uses her Shunpo to jump in like Zhonya's Hourglass 
. Whereas Fiora will want to build Attack Damage (AD) that synergises with her abilities such as Ravenous Hydra 
and The Black Cleaver 
. If you can get enough gold to complete some of these items before your opponents do it will give you a big advantage in fights as you begin to leave your lanes and fight as a team.
and 
. When I'm against a Katarina, I save one of my cooldowns (most people blow their cc really fast in a teamfight when they're new, which allows Katarina and Fiora to have a field day) during a teamfight and wait for Katarina to shunp and ult my team. Then I just hit Q and cherry tap her, interrupting her channel, and my team blows her up.
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is on Kat? Are we trying to turn him from the dark side?
