The Cannes Film Festival is not open to the public. It is an industry event. You get into screenings by registering for the event...
http://www.festival-cannes.fr/index.php?langue=6002
Tickets are not available for purchase.
At the smaller screenings there are lines for different types of participants (i.e. Press, Industry, etc.) A the big ticket competition screenings the tickets are granted through a lottery to registered participants.
There are some tickets available for the people of Cannes as well.
I've been to the Festival a couple if times and it is just nuts... I am in Cannes twice a year for television markets and it is busy... but during the Film Festival the prices all double (food, housing, etc.) and the general population of Cannes swells to bursting...
Let me know if you have any specific questions.