What type of writing?
The needs for a fiction writer over a freelance news writer are very different.
The easiest way is to freelance some articles for magazines, shop around stories/poems.
Check out sites such as E-lance (electronic freelance). They don't pay much but they build your resume.
Check out writer's workshops (for creative). Shop your work to agents and publishers.
For non-fiction, check out blogs, sites and writers on your area of interest. See if you can be a guest blogger or if you can review items related to it. Build your own platform.
Really, to do this, you have to have an idea of what is your defiinition of professional? Fiction or non-fiction? News or art?