If you will be start a blog, it is best you start with a single niche (central theme, specific market) first instead of spreading yourself out to multiple topics and markets.
Start writing with what you are passionate about or knowledgeable. It is easier and more spontaneous writing for you that way. Example if you like traveling, you can establish your travel blog based on point of view of a teenager or young adult. You can revolve your topics from planning, preparation, actual and coming home, and include problem/solving scenarios. It is very important that you will be able to relate to your readers by adding substance to your content by way of bringing pure entertainment, added learning (problem-solving), or upliftiment of the mind or spirit (positive vibes).
Remember a single niche blog will be better off in attracting traffic and loyal following eventually to the blog. Also, SEO is better achieved this way especially in terms of searchability. Monitor progress of your blog if it is attracting its desired market and read comments to get insights for improvements. Soon, you will getting requests to write on special topics which are outside of your zone, where you may decide to evolve and expand to other topics as part of your blog's natural growth process.
If you are a newbie, some try blogging in forums first or work as a freelancer to get a feel, practice and experience before venturing to their own blog. There are a lot of probloggers offering free readings online on how to be start blogging and even bootcamp. Have no fear! Stay focus on your goal and be positive always. Good luck!
