Elise has a somewhat complex kit for a newer player, she is good at applying early pressure but doesn't scale as well with ability power as some other mages, instead relying on magic pen or building tank later in the game.
Gangplank is banned a lot because of his late game damage output, his barrels can chunk enemies for ~75% health, but hitting them can be difficult especially since your opponents can also hit them to disarm them and ruin your combo. You can build Gangplank as a bruiser with a bit more durability and do fairly well though due to his W making him able to escape CC effects and giving him extra effective health.
Katarina is a bit simpler, but has an exploitable laning phase, vs other new players she can definitely pub stomp though as they will be unlikely to be able to capitalize on her weaknesses. You are reliant on getting resets in fights, so be sure you can get the kill before going in. Generally you will want to use Q to farm at a distance, you can max it if you want but I prefer to max W first for wave clear and the speed boost. Save shunpo (her E) for escapes in lane in case you get ganked until you get more comfortable with her.
For newer players I would usually recommend
so you have someone relatively simple for each role.