She says she doesn’t know where to start and I need to help her but I really don’t see why I should
I absolutely understand where you're coming from, especially since it sounds like you're the only adult in the house.
But pick your battles, OP. Is this really the hill you want to die on?
Like many PPs have said, break it down into small steps, and sit there encouraging her and directing, but not doing the work yourself (unless it's something she might not know how to do e.g. using furniture polish.)
Once you've blitzed the room, tell her "room inspection" will be happening once a week and she needs to tidy up before that time every week. Maybe make it a Sunday so she has all day Saturday to get it done. When she comes downstairs and says "Mum I've done my room", tell her to go back upstairs, stand in the doorway, close her eyes, and then open them while thinking "I'm looking through my mum's eyes, would she be happy with the state of this room?"
I used this with my DS when he was 9-10 and it was really effective. He sometimes returned to his room 3 or 4 times! But it really helped avoid him just shoving things under the bed or into a drawer.
Having dedicated storage will help too. Plastic boxes of various sizes from Wilkos, Poundstretcher, B&M, Home Bargains etc are generally pretty cheap, and you can write on each one with a permanent marker or a sticky label, for eg "Drawing/Writing equipment" and put all her pens, pencils, felt tips, notepads etc in there. Any video game equipment can go in another box. Lego in another box. Etc!
I would also suggest rewarding with something speficially for her room after she's done the job. Like picking out some new bed linen or a wall poster.
Good luck OP, this is a really tricky issue when you don't have a partner to support you and it's hard to find a balance between allowing them some indepenence and ensuring they still keep to house rules.