* Je suis gaucher. (This is the most common and direct way to say it.)
* Je suis droitier de la main gauche. (This literally translates to "I am right-handed of the left hand," which is a more formal and literal way of saying it.)
* J'écris de la main gauche. (This translates to "I write with my left hand.")
You can choose whichever one you like best, depending on the context and the level of formality you want.