Hey, it's cool that you're deciding to create your first cosplay and it's totally okay to be confused.
But in my opinion, Katarina seems like a complicated cosplay to create as a first cosplay. With a lot of effort, care, and time it can be done, though!
First off, if you haven't look around on CosplayTutorial.com
, there are tutorials for beginning cosplayers and also great tutorials on how to make larger swords. (You can use foam/paper mache/expanding foam/etc.)
For the outer crop jacket, you can find crop jacket patterns on the internet or just find one of your own jackets and use it as a pattern (just make the sleeves shorter and crop the belly off). I'm not very good with fabric, but something like pleather seems like a good idea (I dunno if it's easy to sew for a beginner though...). Then, like on spcatsasha's cosplay, attach/glue on a belt on the chest of the jacket.
In spcatsasha's cosplay, she just used a black brassiere/bralette instead of a breast plate. It's literally a black band of fabric with elastic sewn on at its simplest, but if you want to create a breast plate, it can be made of craft foam/paper mache. (There are tutorials on the cosplaytutorial.com place for breast plates too btw).
Her spiked belt can just really be a belt you have with small cones glued on them. Or you can buy some studded belts at a store or something.
Katarina's pants are just leather skinny jeans so you can just find a pair of skinny jeans and use them as a pattern. I'm not sure how comfortable pleather skinny jeans can get though... Maybe find some at a thrift store??? Then just sew some pleather bands on her thighs and put the kunai in them.
and for her gauntlets/gloves, it depends on how you want to take them, I guess. You can buy some black elbow-length gloves and chop of the fingers or sew some fingerless gloves yourself (many patterns/tutorials on the internet). And for her arm armor, it can be made with craft foam/paper mache once again. Then just glue on some pleather straps and you can slip them off and on your arms. Any details such as spikes and patterns can be figured out with creativity or just more craft foam.
And for Katarina's boots, maybe find some black boots or create some black boot covers
over some older shoes you have.
Okay, sorry for the long post haha, but I hope I covered everything and helped a lot! Good luck on your cosplay! <3