How can you tell if an avocado is bad?

I think I found some mold in my bathroom recently and am a little worried. I know mold isn't good and stuff like black mold can be pretty dangerous, but I'm really not sure how to tell the difference. Either way, I want it gone sooner than later. Is there anything I can do by myself to clean it up and prevent it from coming back? I'd really love to save some money doing this DIY, but how do I know when it's time to contact a professional?

I've seen so many friends start baking fresh sourdough bread, and I want to make some too. I am so overwhelmed by the process and don't even know where to start. Is there an easy sourdough recipe for a beginner like me? Do I have to get a sourdough starter somewhere or can I make my own?