When you decide to have a blog you must have your reasons. It could be for pleasure, because it is trendy, for earning money, to share with the others things you know, you like, you see. Owning a blog something like a diary. It is yours and it is about you. At the beginning you can write about things that came into your mind. For example, if you love poetry, you can write some of your own or essays and reviews about important poets; if you are an accountant you can write about that; if you are a chef you can offer some good recipes and so on.
The thing is that you have to write about you know, about what you have did researches, about things that you are good at or things that you like to do. It is best this way because you will always have what to write about and with great information and quality.
For good ranking and traffic you need to find out what are people interested in, what they want to find out, what are their needs. Researches about Search Engine Optimization, it is important and necessary, too. If you can get a mentor, it will be just fine, because he will guide you well.
Writing for others is another way to make some money. Writing for companies, magazines, other people, etc it is a good opportunity.
There are more ways and you will find them by yourself. You just begin, start with the basics, write about you like . After a while your material and experience will offer you income and people interested in your work.
Keep it fresh by writing almost daily, do accept and answer to the comments, use ad sense, accept banners and links to other bloggers. Read a lot, write a lot, share and the fine results will come.