Sometimes being in Paris feels like an adventure game.

ride metro
The metro requires a ticket.
buy ticket
From whom?
buy ticket from machine
This machine does not accept your credit card.
buy ticket from RATP agent
"Je ne vous comprende pas. Parlez vous Français?"
jump turnstile
The gendarmes are approaching. Run?

The 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.