As you begin to get an idea of where you want your furniture, imagine you and your family living in it. Do you have a chair for reading that needs to be in a well-lit area? Do you want a lot of room for activity in the middle of the room, or off to one side? Would a table, where crafts and homework could be completed, fit in the room? Is there a TV in the room, and do you want that to be a focal point?
One final thing to decide is: do you want to entertain in this room? If the answer is yes, consider a secondary furniture arrangement to make room for a party. Obviously you won't have a party every day, so it might make sense to arrange your room in a way that allows for easy adjustments for parties.