What should I do to become a pilot for skydivers, in France?
Short answer: you need a commercial pilot's license. The drop zone will provide the extra training, it takes about a day (I worked at a skydive center for 10 years)

However, since you seem to not have any kind of pilot's license right now (or you would know this already) I will add that it isn't a full-time job at any except the busiest drop zones. Our pilots usually worked for a couple of years to build time and then went on to other aviation jobs. During those 1-2 years they had other income - flying for us was mainly a way to log flying hours without having to rent aircraft.