Sometimes being in Paris feels like an adventure game.
ride metroThe Paris metro is actually pretty easy; just buy "un carnet" from the agent, 10 tickets for 10.90€. But living here you want a monthly pass; Here's the walkthrough on how to do it. WARNING: SPOILERS.
Paris has weekly and monthly Metro passes (abonnement). As a visitor you want the Carte Orange (a photo ID card, free) along with a coupon (a metro ticket you reuse). The coupon choices are a coupon hebdomadaire (one week, Mon-Sun, 16€) or a coupon mensuel (1st-31st of the month, 52.50€). You probably want the cheapest ticket, zones 1-2 (for the whole Metro). Residents are being encouraged to switch to Navigo, but that requires a local address and bank account so skip it.
Buying the monthly pass turns out to be easy. Just go to any ticket seller in a metro station and say you want a Carte Orange. He'll hand you a blank photo ID card for free and then sell you the week or months' coupon. Stick a one inch square photo on the card and use the provided adhesive to seal it up. You have to write your card's number on your coupon; that prevents you from sharing your coupon with someone else.
Every city I've lived in has had a different process to buy a local transit pass. In Paris there's no special effort to get the card, you get it anywhere and assemble it yourself. There are photo booths in some bigger metro stations.