A strong sack full of pinecones or whatever, tied to a rope, can be dropped down the chimney to clear loose stuff. Seal the fireplace from the house beforehand to keep carbon from getting all over. It doesn't need to be dead clean to be safe. Make your first fire with dry hardwood and keep it going for long enough to get the brickwork warm. If you're going to use coal, put it down before paper, kindling etc, because it takes a lot of heat to get coal burning properly and what burns first is gas, so you want that going up through burny stuff.