On arrival at Auschwitz you pay an admission fee and will be told what time your guided tour starts as each are according to language (in your case French).
You are free to wander around at leisure but be certain to be at the spot you are told your guide will be on time otherwise you will quickly lose them. It gets very busy. A guide is essential really as it's not only Auschwitz, but also Birkenau camp which is a short bus ride away. The bus transport to Birkenau is included in the admission price.
Whilst there you might also like to visit Plaszow concentration camp which is situated very close to Krakow. Plaszow is best known as the camp where Amon Göth was Camp Commandant, featured in "Schindlers List". The house where Göth lived still stands and when i visited it was almost a ruin, but now appears they have renovated it.
Oskar Schindlers factory is also nearby on the edge of Krakow city and worth visiting. It is now a small museum.
If you haven't been to Krakow before then visit the "old town" part in the city centre.