albert,
I assume you're asking about using a French SIM (Subscriber Identity Module) with your iPhone 5S. A number of people here on the forum have done that, and hopefully they'll provide a few comments on their experiences. I've used a similar method with my iPhone 4S, but I've been using a SIM from one of the "travel phone" companies. The phone number is based in the U.K. but works all over Europe with the same consistent rates.
If you want to buy a SIM for use in France, the best method would be to just buy one when you arrive there. Lebara seems to be one of the firms that is frequently mentioned. If you'd rather use a SIM from one of the travel phone firms such as Roam Simple, Cellular Abroad, Telestial, Mobal or EuroBuzz (there are others), you can buy those online. If you use a SIM purchased in-country, you'll be on a PAYG plan and will have to top-up when your minutes are depleted. If you use a SIM from one of the above firms, many use post-paid billing (calls charged to a credit card), so no topping-up is necessary.
Whichever route you choose, you may need help setting the phone up initially, as some of the data settings may need to be changed.