A great place to start, if you haven't seen it yet, is a post written by Ed Yong of 'Discover Blogs'.http://blogs.discovermagazine.com/notrocketscience/2010/07/29/on-the-origin-of-science-writers/
I would suggest that you start keeping a blog and writing entries in as journalistic style as possible. To get tips on style and format simply Google search "writing tips", and variations there of. It is an acquired skill and takes a lot of practice. Once I felt confident I started sending posts to different places and asking if they'd publish. Or try and "guest post" other blogs etc.
I learned to write by blogging at first, then pitching stories to sources like www.lablit.com
- the editor was very helpful and when I pestered her about helping me get better. She put me in touch with her copy editors and from then it was just a case of writing, writing, writing. My CV has one page for my scientific accomplishments (publications, grants, awards etc.) and 5 pages for "Science Communication".
Finally, immerse yourself in the culture too. Read prodigiously and write endlessly. You'll develop your voice and style and you'll start to become aware of how many opportunities exist (science press officer etc.). But, it is a hard field to break into and even hader to generate revenue in.
My main writing revenue stream is actually as a copy editor for a science database. I sell reviews and articles when I can, and even once got to help write 5th grade textbooks!