The Tale of Accursed Springs
Ranma Saotome was taken from his home at an early age by his father, Genma Saotome, and journeyed with him on a decade-long training trip to perfect his martial arts. Along the way, he encountered many things, and many people, much of which is later revealed as the series moves on.
When Ranma neared the end of this training trip, Genma took him to a training ground in China named 
Jusenkyo. This place, however, was a training ground that no one used anymore because each of the springs that were there were cursed, and falling into them would turn you into whatever drowned there and cursed the spring before. Not knowing this, Genma led Ranma to the ground and attempted to train with him atop the dangerous bamboo poles which jutted up out of the pools. When Ranma knocked Genma into Shonmaoniichuan, or the Spring of Drowned Panda, Ranma was surprised to find that instead of his father, a panda climbed out of the pool and prepared to fight Ranma. 
As a result Genma, now cursed, was able to hit Ranma into another of the cursed pools. Ranma was knocked into Nyannichuan, or the Spring of Drowned Girl. Thus he turns into a girl with cold water, only reversible through hot water.
This curse couldn't have come at a worse time, because Genma had made a certain arrangement with his old friend Soun Tendo. The arrangement was that Ranma would marry one of Soun's daughters, and carry on the Anything-Goes school of Martial Arts. Being now essentially a half-man, Ranma was put in a very uncomfortable position.
Fortunately, Soun didn't seem to mind too much, and engaged Ranma to his youngest daughter Akane. Though truly unwilling to be engaged to Akane at first, Ranma, despite all the remarks he says to her, is actually developing feelings for the short-haired girl. Though he won't
actually admit it out loud, he really cares for Akane and would not want to see her hurt or separated from him.
Ranma and Genma moved into the Tendo dojo, and ever since, things have have never quite been the same in Nerima. Ranma has endured, among other things, Amazons, old rivals, Dojo destroyers, and even evil demons. Ranma's ability to change hasn't helped this situation much, as there have been people, like Kuno who chase after Ranma's girl-form, as well as those like Shampoo, who are after him in his male form...either way, it's double the trouble for our pigtailed martial artist, and Ranma has tried to deal with the action as best as he can.
All in all, whether male or female, Ranma gets into a lot of predicaments. It is his hope that one day he can find a cure for his curse.