A room

My room is on the second floor of our family house. All my walls have white colour.(All the walls are white.) Next to the door there is my bed. Behind(Beside ?) the bed there is a cupboard. On the other side there is a window. Its direction is on the west(.. a window facing west). Under the window there's a desk. Between the second and the third wall there is a computer desk. It's in the corner. (My computer desk is in the corner.) Next to the computer desk there are doors to the next room. There was my sister.(Next to the computer desk there is a door into my sister's room.) Next to the door there is an another cupboard. In an another corner is a TV desk. By the fourth wall there is a fireplace and next to the fireplace there are two cupboards. On the floor there is a carpet. It's brown.(There is a brown carpet on the floor.)

It is easier to form a picture from your description if you mention things as they are located in one direction - for example, start with the door where you enter, look around the room in a clockwise direction, describing what you see. I can't imagine a cupboard behind a bed being much use.

If a cupboard is for clothes, use "wardrobe" instead.

You can start each location with either "Next is ..." or "Then there is .."

You can say "My computer is on a desk in the corner."

It is not clear from "on the other side" if you mean the other side of the bed?

It would be fine if you had previously said "On one side of the bed is a cupboard."