Have an idea as to what categories you are going to have on your blog as well and the overall structure of it.
Obviously, you need to plan with what your blog is going to be like. Plan for your topic and design of your website. Lastly, would be your domain.
The first step is similar to that of writing a book - pick a theme which interests you or along which you think you can keep generating interesting ideas to dish out. A theme helps you to stay focused. Then you can go about reading, research and writing.
Don't forget to introduce yourself to your readers. People like to know that there is a person who thinks and feels behind every post and it's an excellent way to build a rapport with your readers.
There are many blog sites on which we can easily create our personal blog like Blogger.com, tumbler, wordpress, posterous, divatoolbox etc.
You need interest & choose the correct topic and proceed, I am running a free WordPress build service with one premium theme. PM me if your are Interested, I will provide you the more Information.