A: "Paint the walls? Remove the panelling? What color? Also paint fireplace? This room is very dark without lighting. Want it to be cozy. But idk if I like the panelling the way it is."
A: I am excited for the changes you are making to the room! My suggestion would be to paint the paneling and fireplace as the most cost effective option.
Of course you could remove the paneling and redo the fireplace material but this would require a lot of work! Since you mentioned that this room is very dark I would defiantly suggest a light wall color. The floor appears to be a cherry or an oak. I would also suggest painting the baseboard heaters to match the wall color.
I think that a nice light color for all general walls including the wall paneling paired with a white window trim, door casing would look great. Please see reference images I included. The white and light blue paint color will definitely lighten up the dark space. Depending on your color preferences, here are some suggestions that I think would look great for the existing panels.