Qualifications
While there are many art schools out there dying to take your money, the truth of the matter is that a sketch artist does not need a degree. Learning something might be fun, personally fulfilling, or useful as a back up plan; but for the most part, a really good drawing of a degree is likely to get you farther than an actual one.
That's not to say there aren't some pre-requisites for the job. As a freelancer, you're going to need some basic math and business skills, which you can either take a community college course in, or just learn through experience.
You'll also need really great people skills—extracting the description of someone's attacker the day after they were attacked isn't going to be easy. Oh, and we guess you should probably be good at drawing, too. So, there's that.
You may also want to go the extra mile and get certified in forensic art if you're going to be looking for that sort of work—the International Association for Identification has a pretty good program—or hunt down some formal training in facial imaging software. After all, the best way to combat losing your job to someone else is to make sure you are that someone else.