You should search for images you like and use them for inspiration. If it were me I would paint the walls a neutral colour avoiding yellows as I think the brick ceiling and tiles floor adds enough warmth. I would choose a nearly white with a hint of brown/grey. I would paint the windows and doors a dark colour - I would choose a nearly black with a brown quality. If you decide to keep curtains I think natural linen or hessian is a simple rustic choice that fits the architecture. For me the bricks of the fireplace don’t have the provenance of the ceiling or floor so I would paint them a soft white.. I would likely paint the inset shelves above the fireplace in the wall colour and then use them to display beautiful objects or a lamp.
If you can make the glass door outward this would improve the available space and access to the light switches. Personally I think the kitchen tiles are too plain and a patterned tile would look great. Personally I think the kitchen shelf is too high- I am sure different brackets would allow this to be hung lower- since it’s a tall room you could add an additional shelf for visual interest.
Furniture- in my opinion you need one large cabinet or desk to sit under the window to the right of the fireplace. I think you could put a chair near the radiator and have room for a tiny settee near the fireplace. I would rather see plain furniture and have use artwork to lend pattern to the room. I would put the TV on the wall to the left of the glass door and put a free standing screen in front of it when not in use. I would move the coat hooks to near the double doors or get a coat stand instead, then upgrade the fridge to something more stylish. Lastly I would add a large circular rug to help stop echos and additional lights which will be nicer at night than the ceiling lights alone- I would choose an uplighter to bounce light off the ceiling to the right of the fireplace and above the kitchen shelves too.