Choose a fictional world that will appeal to you and that you won't lose interest in. One easy place to start is with a world that has already been created--that way you have a landscape, characters, and a history pre-made and ready to build on. If you are a fan of the Star Wars Galaxy or the Gilmore Girls TV show, you could choose one of these worlds as a framework into which you can insert your blogging persona. Or, if you want to make up your own world, spend some time thinking up the rules that you're going to play by before you start writing, so you can narrow down the scope of your project so that it's more manageable.