If you are just venturing into the world of freelance writing, there are some things that you should do to prepare yourself.
The first thing you should do is figure out how long it would take you to research and write an article, both 300 words and 500 words. This will be just so you can know how many articles you would be able to write in an hour. Make sure that you give yourself enough time when applying for writing jobs. I had a job offer today where they were expecting me to write 4- 350 word articles in an hour. Yeah, I'm sure I could do it, but they would be shotty articles because I wouldn't have the time to do the proper research and proof read everything a few times.
The next thing you should do, is figure out what the smallest amount is that you are willing to take for your time and effort into writing an article. I am bringing this to your attention because I am seeing too many writers selling themselves short. When you are applying for jobs on freelance websites such as elance and odesk, be sure to know what your hourly rate would be. I've seen job posts where some clients are only offering $1 for a 500 word article. Seriously? My time is worth a lot more than that, and if the topic is something that needs researched, it is going to take even more time to search and read through material. I can't believe that so many people are willing to work for such a tiny amount, or that clients or buyers expect writers to put all kinds of work into an article, for mere pennies. If you are a good writer, you are worth more money!
If you are going to try out one of the freelance websites I have posted about, make sure that you know what to say when you apply for a job. Unfortunately, there are lot of other writers out there who are applying for the same job and are willing to work for pennies per article, however their articles may not be of very good quality. When you apply for freelance jobs, be sure to list any experience that you have, whether it be blogging, guest blogging on other blogs, other freelance writing gigs you have done, any college courses you may have taken, and what your areas of expertise are in. Also, always make sure to give them an example of your work. If you have never done any freelance writing, go practice by writing a few articles. Pick a subject and then write about it. Always have something to show in your portfolio. Most freelancing websites provide a way for you to upload articles to put in your portfolio or profile. Also, if the job listing includes what subjects they need articles about, or how many hours per day, be sure to include something of the same nature in your application letter.
Also, never write a specific article for someone as a test. I've had people contact me telling me, "write a 300 word article on Ohio Real Estate (ok, I made the Ohio Real Estate thing up, just as an example), for a test and then I will decide if I want to hire you." Errrrrrrrrr, hit the brakes! This is a great way for them to get a free article out of you. Don't waste your time on something you don't even know if you are going to get paid for.
Once you begin producing quality articles, clients will begin noticing you. Don't expect to get a bunch of jobs immediately, it may take time; just be patient and write your butt off!