In freelance writing I always had the most success (i.e. publication) when I wrote about something I actually did. I sprinkled in applicable anecdotes of my own experiences with the text to make it more interesting. As an example, I submitted an article to Careers magazine about my job when I was working as a student in the Transportation department of our local hospital (OHSU). Always keep your target audience in mind in terms of subject matter and writing level (i.e. sixth-grade-level writing, college-level writing, etc).
Your hardest part will be the break into print. If magazines thrill you, you could get a start by going to a restaurant or watching a movie on opening night and writing a review. There are always local "free" rags that might accept your article cold and put it into print. Don't expect to get paid more than a pittance for it, but it might give you an idea of the quality of your work and get you started with that first break. Once you're in print, you can attach these clips when you want to write a more "targeted" article for a bigger magazine.
Novel/Novella writing is a different animal, though. You'd need to be a bit more specific about what you ultimately want to do.
IMPORTANT: Don't get caught by some lousy "Vanity Press" where you pay them to publish your work. That's not how the (real) writing business functions. Also, make sure you always have another source of income. For every starving artist, there are three starving writers 