What do I need to do before I can adopt a child? Can I adopt without having a spouse? What quests do I need to complete and what do I need to build? Thanks.

Need to kill the old rhymes-with-witch who runs the orphanage (kicking off the DB questline... though you can quickly put an end to it without having to do the quests.) Then, you need to buy/build a house that's added by Hearthfire, and then build the children's wing. And then you can talk to the new nice lady who was the assistant to the old rhymes-with-witch to adopt.