The skinny and much much more
This means that you would be a far more effective blogger if your blog focuses on a subject that you are intimately familiar with. While no one is expecting you to know every single thing there is to know about your chosen niche, you should at the very least be in possession of a bit more knowledge and/or experience than the majority of your readers. Remember that your audience is will want to pick up some information or unique insight on that particular subject matter that you're focusing on, and having some knowledge is the best way to provide that content. Ripping off other people's content is a no no. If you have nothing at all to write on your blog, resist the urge to lift content from other blogs.