How do you choose a good topic for a blog and how should you write it for it to be successful?

What elements are necessary for making a successful blog and what are the unwritten rules of blogs? Should it include a goal and timeframe that you announce to your readers? In addition, what should you do if you already started that goal in January but want to start the blog now? Is it necessary for the blog to be written in the present tense or can I use past experiences and write them as if they were happening today? Should those be written as flashbacks, or can I start now with past experiences and advance until I catch up with the present?