If you're looking for a self-defence class and you've never taken anything like one before, you'll need to be aware of how "self-defence" differs from martial arts. You may come across classes that appear to be a combination of the two, or you'll see ads for martial arts schools claiming they teach self-defence. Some really do; they have separate classes. But sometimes it's not so easy to figure out which is which if you're not already familiar with styles of martial arts. Note that this isn't meant to dismiss martial arts, which are wonderful. But knowing more about the differences between the two will help you find a class that is appropriate for your goals.
Survival and Evasion vs. Self-Control and Synergy
Self-defence and martial arts share a lot of characteristics, and in fact, much of what you learn in self-defence classes can be based on a martial art or arts. And, of course, much of what you learn in a martial arts class can be used for self-defence. The main difference is that self-defence is geared toward surviving a fight and evading an attacker — it's about getting away and living to tell the tale (and the police). Martial arts teaches you to fight but also how not to fight at those times when controlling yourself and extracting yourself gracefully would be a better option. Martial arts also takes synergy into account, the overall combined effect of everything you've learned to form a whole system.
Both self-defence and martial arts have varying training times depending on the system. Some self-defence classes are only a day, or maybe you have seminars once a week for a couple of months. Other self-defence classes are full defence systems themselves, taking years to learn all of the moves and often mimicking martial arts classes in ranking structure. In these classes, you learn basic techniques at first, and then move on to defense against weapons, for example. With martial arts, some promise to make you an effective fighter in maybe a year or two, but even then, you can continue on for years to reach the highest ranks, learning things like pressure points and some alternative medicine along the way.
Self-Defence Isn't Competitive
A very big difference is that self-defence isn't competitive. These classes aren't built to create competitive fighters (those in the MMA world are an exception, although that system isn't technically self-defence or martial arts; it's more a competitive sport based on martial arts moves with the ability to use the moves for self-defence). Martial arts schools often are part of systems that hold competitions. This is not always the case, of course, as many smaller "family styles" don't really compete.
If you want a little training just so you know what you could do in a situation, a day class or series of classes short-term might be what you want. If you want to become an effective fighter for self-defence purposes only, then one of the longer-term systems may work for you.
For more information on self-defence, contact a professional near you.