1. What do you really want to do? Start with what you absolutely know you would love, and narrow down the focus to get to a clear picture of what this job means. A big picture focus would be "a teacher" - a more specific goal is "a science teacher at a magnet school in the city."
2. What are the reasons you want to do it? Sticking with the above example, a good teacher wants to teach because he believes in education as a value, he thinks he can be a positive influence on a child, he wants to help his community, he has a depth of knowledge in a certain field, etc.
3. What do you get out of it? This is really about what you get out of it - the things you really value. It's not always about money or prestige - a teacher might not make as much as a lawyer, but he has summers off. This is the time to consider how what you get matches up with your values. If your real dream is to spend more time at home with your family, a high-stress, long-hours, high-income job probably won't make you feel really satisfied.
4. What do you need to get it? Maybe you need a certain degree or certification. Maybe you need a really good suit. This isn't about all the things you can get after you make your career change - this is about the immediate things you need to position yourself as a highly competitive candidate.
5. Who can help you get what you want? Now that you've considered the above four questions, think about who you know who has done something similar, or who you know who knows somebody who has done it. You won't know if you don't ask - tell people specifically what your goal is so they can have a better idea of what's involved. Using social media to pass information around at this point is a great way to expand your knowledge base. This step naturally leads to very useful networking - becoming associated with a community of people who are in your target field is the best way to find a job you'll love.