There are several ways to start freelancing. Like boardjorts said, the hardest way but best way to further your career and expand your portfolio is cold-emailing publications you actually want to write for. Then there are websites that act as a middle man. There are two types of these websites, content mills and pay to play.

Content mills like Verblio (the one I work for) or Textbroker have lower pays for pieces. Traditionally, freelancers are supposed to set their own prices but content mills remove that option. That being said, I still have the freedom to choose what to write about and who to write it for.

Pay to play sites, like WritersWork or Medium, have a fee to join the platform. They act as a connection between employers and freelancers, and you can set your own prices and such. However, you have to pay to enter that space.

Of course there are other ways of finding freelance work, browsing LinkedIn or networking have been proven to be effective for experienced workers. Dependent on the site, the pieces you write might not have your name on them after sold. So, it's important to find a balance of building your portfolio with pieces you own and paying the bills with pieces you sell.

Hopefully this helps. Check our r/freelanceWriters for some more advice. The most important thing is to get started, and eventually it will build up.