This is a question I get asked a lot. In fact, I even asked it myself when I first struck out on my own. The experts all say to make a list of the things you like and the things your good at, then choose something that's on both lists. You'll be much happier and more likely to succeed if you do something you enjoy AND something you're good at.
In the book, "Outliers" by Malcolm Gladwell, he suggests it takes 10,000 hours to become an expert at something. I don't necessarily think you have to be an expert at something to start a business, but it helps to have a good understanding of it, and the better your understanding the more likely you are to foresee roadblocks and obstacles.
The natural progression for most successful entrepreneurs starts at a job getting paid to learn the ropes, and then leads to self-employment doing what you spent years learning. For me, that was web development. I spent years consulting for other companies before I started my own successful consulting company. Then I launched a website that related to my experiences as a consultant, which I later sold. I believe this natural progression is what led to my success. People who start a business that has little or nothing to do with their past experience have a very difficult uphill battle ahead of them.
My final piece of advice is to start your business on the side (meaning keep your job) if at all possible. This will also increase your chance of success by allowing you to continue paying your bills stress-free until you earn enough from your business to quit your job. People often over-estimate their revenue projections due to optimism and enthusiasm (I certainly did), then they make personal financial decisions based on faulty projections and that can be really detrimental to the business and your personal finances.
Being an entrepreneur is an awesome experience. Good luck!
JP Stonestreet
www.WebToRich.com
Don't make a living...make a million!