To start a WordPress blog, you need to first Identify your blogging niche. Then you need to buy a domain name and hosting.

If you do not want to invest at first you can also start blogging using wordpress.com, where the domain will be like www.yourblog.wordpress.com.

This is good for new bloggers, to see how well they are getting responses for their content.

But if you want to start a wordpress.org blog you can follow the steps here

In this link, you will find all the step from start to end

Let me know if you have doubts