Depends on where you're starting out. Are you a newspaper journalist? Or are you currently writing for a trade pub? Email me -- maria@hypergridbusiness.com -- and I can give you more specific advice.
If you've got your freelancing cleared with your boss, you should start out by targeting publications at roughly the same level as your current market and work up from there. If you're at a newspaper, however, this won't earn you very much -- you're better of building a tech or business niche from scratch.