Robert
I've been into Karate for too long to drop it for another style, but if I was going to I would choose Krav Maga.
Go on Utube and check out some of the instructional and introductory videos for several of the different arts and choose the one that you most like.
It is possible to learn some through books and online courses, but taking actual classes may produce better results. (Depending on how good the instruction is).
It's really going to depend on how much time and effort you are willing to put into it.
Mr Monster
Krav maga. Not just because of the quick and efficient self defense it teaches to win a fight, but it teaches how to avoid a fight altogether with situational awareness and verbal deescalation.
u_bin_called
I know certified instructors in a number of martial arts and all agree that the #1 "art" to learn for self defense is awareness. Being aware of your surroundings, potential threats, avenues of escape and your own behavior will go a long way in preventing you from being a victim.
When asked what technique he'd use in an actual street fight, Chuck Norris himself said "I'd run. That is the single best defense. Fighting is only if you have no other choice."
That said, if you are new to martial arts I'd suggest starting with good old Western Boxing. In my opinion it will give you the best starting base, not just in striking but in understanding the importance of stance, distance, timing, mechanics and conditioning.
Steel
Now, the world don't move
To the beat of just one drum,
What might be right for you,
May not be right for some.
?
1.Brazilian Jiu-Jitsu
2. Muay Thai (Kickboxing)
3.Boxing