Post offices are signposted La Poste (www.laposte.fr). For a proper stamp (un timbre), rather than an uninspiring sticker (une vignette) from coin-operated machines, go to windows marked toutes opérations (all services). Tobacconists and postcard shops also sell stamps.
Postage rates are calculated both by size and weight. Standard letters weighing up to 20g/100g cost €0.77/1.54 sent within France, or €1.18/2.36 to Zone 1 (EU, Switzerland, Iceland, Norway) and €1.29/2.58 to Zone 2 (rest of world).