Choose wisely. If this comes off, you might be writing about this subject for the next 10 or 20 years! The good news is there are tons of amazing things to blog about.
What do you love to read about, talk about and do in you free time – is it cooking, travel, golf, running, chess, fashion, beauty? What could you see yourself writing about for the next 10 years or more?
What expertise do you have? What do your friends say you are really good at? What does everyone always ask for your opinion about? Are you amazing at putting together a great outfit? Getting your kids to eat vegetables or go to sleep at night? Cooking on a tight budget? Being organized? Or perhaps you have a special skill or qualification? Are you a finance expert? A nutritionist? A trained lawyer?
Successful blogs solve problems. The bigger the problem, the more likely your blog is to be successful and the easier you will find it is to make money with your blog.
Big problems that need solving include: how to be healthy/lose weight/go vegan or gluten free, how to save money/living on a tight budget, how to get rich, how to look good, how to be organized and productive at work/at home, how to get kids to sleep/eat/behave.
If you can teach people how to do one of those things, or something similar, you are onto a winner!