When people decide they want to start blogging they are often left clueless as to what to blog about. You can blog about any topic you want, but that doesn't mean you should.
If you are looking to make money with your blog, you need to have a tightly focused blog. You will want to blog on one subject that you have a lot of knowledge. For example, if you know a lot about modding the X-box you could share all you know. If you do this to make extra money you could give enough information so that people find your blog useful, but leave enough out that they still need your services.
If you don't care about making money and just want a place to vent or share your feelings, you can blog about anything. The main key to deciding what you want to blog about is deciding what you want to accomplish with your blog.
Are you going to be using your blog to promote a service or product you offer? What are the goals you want to achieve from blogging. If you want to use your blog for marketing but still would like a place to talk about your life, you can have more than one blog. I wouldn't recommend venting about your love life on a blog you are trying to promote your business. It doesn't look professional.
So first decide why you want to blog, what you want to accomplish with your blog, and then finally you can choose what to blog about.