From the article on Christian Post:
he second season of the show has already been confirmed and production has already been underway since August 2014, however, the precise release date for the next season of the show has still not been officially confirmed yet.
Also in August, an OVA series was announced, which will be based off the prequel manga called "Attack on Titan: No Regrets" and will consist of two episodes.
The OVA is scheduled to be released alongside the 15th and 16th volume of the main manga series on December 9 this year and April 9 next year.
Aside from the new OVA, the first season of the anime will be compiled into two anime theatrical films which will feature new voice acting from the same cast. "Attack on Titan Part 1: Crimson Bow and Arrow" will cover the first 13 episodes and will be released on November 22 this year.
The second film, "Attack on Titan Part 2: Wings of Freedom" will cover the remaining episodes and will be released on 2015.