In most cases, you will be best off starting a practice area in something you understand. If you are a litigator, then do litigation. If you are a corporate attorney, then do corporate. If you are a patent attorney, do patent work—and so on.
Ideally, you should operate in a practice area that is in demand from people and/or small businesses that are most likely to hire you. Within your practice area, you should do the sort of work that large law firms will not necessarily do and in which you have a skill that is “niche” and other people in the market are not doing. If you are in a large geographic area with lots of attorneys, you will be better off specializing. If you are in a smaller geographic area without as much competition, you may not need to specialize and can do all sorts of work.
At one point in time, I considered becoming a solo practitioner in Los Angeles, and I investigated numerous practice areas. I wanted to do something where I thought I would have the opportunity to continually make money and would get endless amounts of business. The practice areas where I thought this was possible included family law, representing drunk drivers, regular commercial litigation, suing businesses for environmental violations, and a few others. Unfortunately, I was not interested in doing any of these practice areas.
The most important thing is that you are interested in whatever practice area you choose to specialize in. The more interested you are in the practice area, the more your passion will come through—and the better you will likely do once you get started. You do, however, always need to be aware of income and the need to keep it constantly flowing in.