The first thing you need is a domain name. Your own address.
Then you use Wordpress to build a site at that address. Its technical but not too difficult.
Then, the hardest part of all, you've got to get people to visit the site. And there's ways of doing that, Facebook and twitter drive traffic to your site for instance. Posting here too.
It takes time though, and patience. But most things do right?
Its possible to make a blog hosted on a free site instead of getting your own domain but in my opinion you should get your own domain. If its successful you'll want your own domain anyway and you wont want to change over later. And if you're using a free host then you're not really serious about it and it'll probably lapse.