The most important thing is to start blogging, if you just think about starting it, it will never become famous! So, first of all, go ahead and start your blog today. Second of all, you must have something interesting to say. Remember that everyone's taste is different, so what is interesting to one person might not be interesting to someone else, but usually.
So write from your heart or about things that you like. If you try to create a blog that isn't you, it will soon show and you will get tired of the blog you are writing. If the content of your blog is a niche of some kind it is also good, it means you will have less competition.
Basically, you should write about things that you like to write about. The third advice is to write at least a post every day so that readers who like your blog don't get disappointed when you're not updating your blog regularly. A smart thing when you have some time extra, is to write several posts that can be used those days when you are out of time. For example, you could write seven posts on monday and just publish one of them that day, then the work is done for the rest of the week!