The prequel will tell of the origin of the character previously played by Patrick Stewart -- as, of course, a much older Charles Xavier (aka Professor X) -- as well as his archenemy, Magneto (previously played by Ian McKellen). No word yet on the casting for the latter role.
Here is Fox’s full summary of the movie, per The Hollywood Reporter: “First Class” will “chart the epic beginning of the X-Men saga. Before Charles Xavier and Eric Lensherr took the names Professor X and Magneto, they were two young men discovering their powers for the first time. Before they were archenemies, they were the closest of friends, working together, with other Mutants (some familiar, some new), to stop the greatest threat the world has ever known. In the process, a rift between them opened, which began the eternal war between Magneto's Brotherhood and Professor X's X-Men.”
Source: The Hollywood Reporter