Getting a local sim-card in Paris is often more difficult than getting a sim card in other cities.
If you want to use your phone with local sim-card, you need to make sure your phone is unlocked and supports GSM. Or you can purchase a pre-paid package that includes a basic phone (it will not be a smartphone).
Orange is one of the largest mobile operators in France, and they have a package called "Mobicarte". You should be able to buy one in any Orange retail outlets, however they might not be available in every Orange store. Try those Orange retail stores that are close to the center of Paris or next to the major tourist attractions.
Another carrier is SFR. As with Orange, try those SFR stores that are located in Central Paris, and especially the ones close to tourist attractions.
It is also recommended to print out a page from websites of Orange or SFR with the plan and type of card you need so that you can show it to the mobile carrier representative.
The words you need to know:
La carte prépayée - pre-paid card
Sans engagement - without a contract
Make sure the sales assistant at the store helps you activate the card as otherwise it might turn into a not so fun adventure, especially if you are not fluent in French.
If you need a data package, there are multiple options, but explaining what you need may get tricky. Similar to the phone prepaid plans, ask the sales assistant at the retail store that you need a prepaid data plan without a contract, La carte prépayée pour internet, sans engagement.
Because there are so many data plan options, after you purchase one, make sure there is in fact no monthly charge as canceling the data plan after you get back home will be a challenge.