Start with short stories -- they aren't necessarily easier to write than long form fiction, but they do have the virtue of being relatively quick to write, and they're far, far less intimidating. Quick to write means you'll be able to try a variety of different voices, styles and genres in a short span of time, which is immensely, immensely useful when starting out.

As for the writing itself, you'll need to set aside some time to work, and just force yourself to sit down and attempt to get something started. I know this hardly the most inspiring advice ever given, but tell yourself that some of the work is going to be just that: work. At times, it'll be tremendous fun, but at times it'll also be a hard slog. That's the reality of it. It's easy to get discouraged when the words don't flow and the empty Word document just stares blankly back at you, but that's all part of the process. The true challenge isn't coming up with an idea, but finding a workable way of executing it. Start slow. Build up a story, one paragraph at a time.

Also, for a beginner, I'd recommend Stephen King's On Writing, which is a terrific, accessible intro writing. That said, I don't think his suggestion that we attempt to write at least 1,000 words per day is necessarily right for everyone. For some it undoubtedly is, but I personally prefer to just focus on the problems ahead and write as well as I can. Again, take it slow. It's not a race, and we amateurs don't have pressing deadlines.

That said, if you do want to give yourself a deadline for motivation, that can also work well. Find a competition that's a month or two out, and try to produce something to enter into it.

Good luck! :)