At its heart, blogging is about sharing your knowledge with the world. Choosing a topic that you are passionate about makes the process of starting a successful blog so much easier. Writing about more than one topic is totally fine too. As long as you are writing about things that you are genuinely interested in, your passion will shine through and keep your readers interested.
Working with advertising networks isn’t your only option when it comes to selling ads. If you end up with enough traffic, advertisers may come directly to you and ask you to place their ad on your site. You can also contact advertisers yourself. The biggest difference from the above mentioned option is that there is no middle man, which means you can set your own ad rates.