I am a freelance web writer and now have private clients. However, in the beginning, I signed up with various agencies, some of which are known as "content mills." Unfortunately, the pay is incredibly low, but they are a way to learn some necessary skills. I can recommend Boost Content, Great Content, Copify, Textbroker and Constant Content. Always be aware that you are expected to pass spelling and grammar tests, plus write an article on a topic of their choice, before being accepted.