I started out using Odesk, but that site is full of people willing to work for basically nil these days. There are alsos sites such as Elance, which is similar, but usually has better pay options. Textbroker is another site, but again - it does not pay well until you do several low paying jobs to earn access to the better paying ones.
One of my favorite sites to freelance write on is WriterAccess. They require you to take a test to decide what level you start off writing at, but you have the opportunity to move up in levels, and pay is never below a penny per word.
Then there are various sites on the web similar to Craigslist where people post ads seeking writers.