If you plan to pursue writing and specifically, freelance writing as your sole source of income, you need to write a business plan. Joining the ranks of the self-employed can be empowering, but you don't want to undermine that inspiring energy by simultaneously becoming a starving and harried artist. The way to prevent this is to plan for a successful transition from your traditional job to being a writing entrepreneur. Learn more at Bplans.com.
Note: This is not an endorsement of the software offered at this site, however the articles provide a good starting place for people new to business planning.