Unless you will be using your blog as a personal journal or expression of yourself, you really need to choose a topic to blog about. If you want to blog about more than one topic, thats fine, but ideally, each topic should have its own blog. You can easily set up multiple blogs, this isnt a problem. You could also set up one blog with different categories set up for each topic that you are interested in blogging about.
With that established, you need to determine what it is that you will blog about. Keep in mind that there are no set rules concerning how long or short your blog posts have to be. What is important is what you are saying, not how long and drawn out the post is, or how short the post is. If you need a number just for your own peace of mind, lets say that a blog post between 300 and 500 words is sufficient, but it can be longer or shorter if you want it to be.
It is a bad idea to try to blog about something that you have little or no interest in. Not only will blogging become a chore, but you will quickly lose interest and most likely will not stick with it. Blogging should be enjoyable or at the very least satisfying even if you are doing it for financial or business purposes.
Choose a topic that you either are passionate about, or that you could become passionate about as you learn more. As you can see by this statement, you dont necessarily have to blog about topics that you are an expert on. You can blog about things that you have interest in, even if you dont have an abundance of knowledge on that topic just learn as you go. In fact, this will make blogging even more interesting and enjoyable for you.
Right now, make a list. Think about the topic that you are the very most passionate about. Is it relationships? Politics? Religion? Gardening? It doesnt matter what it is, as long as you are passionate about it. Now, with that topic in mind, start making a list of the things that you could write about that topic. You can call this a sub topic list if you like. Try to list at least ten things, but feel free to list more if you can.
Now, save that list. Once youve started your blog, you already have a list of topics to blog about. You wont always take topics from the list, and that is fine you can save the left over sub topics on your list for those days when you need to update your blog, but cant think of anything specific to blog about.
Use other resources for blog post ideas. Read other peoples blogs. Visit websites that pertain to your topic. Visit related forums and see what people are talking about that relates to your topic. Ideas for posts are everywhere you just have to seek them out.