Citing your sources is one of the most important things you must do as a genealogist. Citing your sources accurately will let you do the following:
Evidence!: Citation & Analysis for the Family Historian by Elizabeth Shown Mills (Baltimore: Genealogical Pub. Co., 1997) is the best single manual on modern citation practices in English for genealogists.
Although there is no similar book written in French specifically for genealogists, any French-language guide or manual on genealogical research and methodology will describe this important practice.