To be a good French translator, you should also know the language you translate into very well. Without knowing the target language, your French Translation definitely would have many structural errors. Further more its intonation, localization etc., would not be at the accepted level.
To become a good French translator, you should learn the language by attending formal French classes; you should be an avid reader of French books as well. Looking at French movies and talking with French people also help. Learning a language cannot be limited to a particular time duration, but it should be done throughout your life.
If you want to become a professional French Translator, you have to follow a French-translation oriented certificate course, undergraduate degree, and further qualify with a master level status in French linguistics.
The organizations such as the American Translators Association, the American Literary Translators Association and the National Association of Judiciary Interpreters & Translators have qualified French Translation workers who have full membership in them. If you want to be an accredited French Translator, you should also join one of them. They organize many national and regional activities, which give you extensive exposure in the field.
You can also attend conferences of translators, and learn from them linguistic techniques. Exchange your translated French documents with fellow translators and compare theirs with yours. You can also get help from computer-aided programs such as DéjàVu, Across, WordFast and Trados. They are rich with stored translated phrases; idioms and words that would immensely help improve your translation skills in French.