Getting a Cell Phone
Getting a cell phone is super easy. This is one area where the French love of paperwork does not really come into play.
You can get a prepaid SIM card but those things are a joke. They're expensive and limiting if you have a smart phone because smartphones eat data like its crack. You're better off just getting a contract.
To get a contract is easy enough. Decide if you want a new phone, or if you just want a sim. If you're an English speaker, buying the phone and setting it up over the Internet is much easier (that way you get to read the fine print). To do so make sure you have your carte de sejour number, your credit card, and your banks RIB. That's all you need. If you just want a SIM the best place to go is probably the one and only Free store in Paris, located at Madeleine. Apparently they have commande machines that give you the sim then and there (you just need your credit card to sign up the monthly payments).
Unfortunately my bank being SoGen, and therefore French, so inclined to naturally give me a hard time, did not tell me I needed a pin to make purchases with my visa online (you get the pin on your cell, France is nothing but jokes and irony) so I had to derail my plan of easily buying it online and go into the store. I went to the SFR near Saint Paul. He set everything up without saying more than a few words to me.
One thing I did ask him about was the 100€ reimbursement for the phone that I should receive later but I ended up never receiving it, despite him assuring me multiple times I would! So make sure you make him write out, in blood, that you will receive it later on. Other than that I had no problem. This is one area French bureaucracy doesn't make you want to poke out your eyes with hot pokers.