One of the many pieces of advice that veterans have for new freelance writers is to specialize what they do.
When you first start out, you may find yourself casting a wide net, willing to write just about anything someone will pay you for. That's how I started out. I was writing for pennies on topics ranging from diets and fitness to Halloween costumes to mixed martial arts gear. It's how you build a portfolio and gain experience in freelance writing.
As you write, though, you may find yourself drawn to a particular area of writing. For example, I found myself writing a lot of health/fitness/natural living articles, as well as a lot of education articles. My background working in education gave me a good foundation for education articles, and with my mother working as a nurse, I knew enough about health/fitness to break into the topic, and built on the initial articles to write more.
Though I still write a lot of different things, depending on what clients need, I tend to stick to health/fitness articles (which includes nutrition, the field of health care, exercise, natural living, etc.) and education articles. These are the two areas I have the most experience in, and can write them pretty quickly with minimal research (depending on the specs, of course).
By specializing, I've been able to strengthen my portfolio. I focus on these two areas, so when a client comes up and asks for samples, I have recent pieces ready to send. I won't say I'm an authority in those areas, but I'm confident enough in them that I can tell clients that there's no problem writing about virtually anything that falls under those umbrella topics. (In fact, I have one client that asked me to do a project that consisted of many topics covering nursing education--it was the best of both worlds!)
Specialization does not limit your writing. Sure, you may find yourself passing on projects because they aren't your niche, but it means that as you grow and strengthen your skills, you can command a higher rate for the work you do. Instead of settling for a lower rate of pay and casting a wider net, you can focus in on the "big fish" in your topic areas. In addition, writing what you know or are comfortable with is more cost-effective. If I have to write about a topic I know nothing about, I'm going to spend more time doing research, which means the article takes longer, and I'm getting paid less per hour. If I stick to the areas I'm comfortable with and know well, I can write faster and be earning more for the same amount of time.
If you haven't specialized, you should consider it. Think about what you like to write, what you're good at writing, what you know a lot about. Think about what you did in your life before writing. This can help you narrow down your writing and strengthen your skills.