I started out as a freelancer writer when back in highschool some three and a half years ago. I had no clue where to find clients, how to market myself or anything else about business for that matter.
But I knew how to write.
I started out with Elance, Freelancer.com and Fiverr (none of which I use anymore really, though Elance is now Upwork and I sometimes gothere just to see if anything interesting is on offer) and just applied to every single job that paid and promoted my Fiverr gigs.
After I slaved for some months on low salary I just got tired of it, and decided to get paid what I was worth to the client with my writing.
I didn't get many clients after that, but the few regulars who kept coming back for more and more content proved it was the right call to make.
And now I've had a BST here for some time which was probably the single best investment when ROI is considered that I've ever made (writing wise that is).
My advice is to contact companies, bloggers and anyone you think could use quality content (don't spam PMs on BHW though...) and ask them if they'd like to post some of your content. For free.
What's the trick with that then?
You start your own website, an online CV basically, where you explain who you are and what you do, and how you can help brands all over the world by writing some awesome content to them.
The free articles you wrote (don't ghostwrite them for free though!) are guest posts, and can seriously help you bringing in some traffic to your website.
From there on out, it's just a matter of closing those visitors to your site to turn them into clients.
There are dozens of excellent guest posting articles online, just Google them.
Make sure to keep the content relevant to what you do, for example write about "How amazing content can really help you grow your business" or "7 Magical Traffic Acquisition Tricks" and speak about how content and content marketing works wonders.
If you need any further help, don't hesitate to ask mate.