How do you say arms in french?

well, if you mean the body part, it is bras, but if you mean like guns and stuff, armaments, it would be materiel de guerre, or you could also say armements.

~J'aimelefrancais:)

OR more simply "armes", as in the rally call "Aux armes!" (to your weapons) or "arme-a-feux" (firearm)