Professional bloggers earn by writing about something that interests them and frequently posting those writings to a website log -- a blog. They make blogging pay.
Does the topic have to be something you are interested in? Technically, no. You can blog about anything. But, will you? To be a professional blogger, you do need to be making a profit doing it. And to do that, you typically need to write and post often. If you aren't interested in your subject matter, just how often are you going to write about it?
Choose a topic that interests you. To increase your readership, and thus your income, you need to post frequently. That's easier to do if you enjoy reading, writing, and researching about your topic.
At first, you'll likely be the only one reading your blog. You might as well write something that you'll enjoy. Make it personal. Share some of yourself. In the writing world, that's sometimes called "finding your voice." Anyway, if you are regularly writing interesting posts about something you like, you'll probably start attracting others like you. They'll also be interested in your subject matter, enjoy your blog, and tell their friends.
During the meantime, your frequent blog posts should be attracting search engine attention. When you get listed in Google, Yahoo, or the prominent search engine of the day, you'll get more readers.
Readers equal revenue. And, there you are back to what makes you a professional blogger, as opposed to a hobby blogger -- profit. The professional blogger has an income coming in from his blog.
That income can come from a variety of sources. Affiliate sales. Google Adsense ads. Selling a product or service. Referral income. An employer who pays you to blog.
To be a professional blogger you need computer and Internet access. You'll need a blog host, which can be either free or paid. And, you need self discipline to write frequently and keep at it.
I use and personally recommend SiteBuildIt! as a host and blogging program. I suggest that you look at what SBI! offers and compare it to other blog and website platforms. If you are just going to play around, it probably offers more than you need. But if you are going to seriously build a website or blog to bring in an income, then you'll probably find it a good deal. It has many features built in, so you don't need to buy a bunch of things piecemeal and so you don't need to learn how to do a lot of the technical behind-the-scenes work. Find out more at about SBI!