Move to Los Angeles, California. If you're serious about acting and being cast in a Warner Brothers movie, you must go where movies are being made. Casting directors will not come to you.
Gain acting experience. You can do this before and after you make the move to LA. Your chances for being cast in a Warner Brothers movie will rise if you can demonstrate that you have acting experience, even if it's just at the local level.
Hire an agent. When looking for a talent agent to represent you, you need to be smart and read throughly into the agent's credentials. Talk to other actors who've signed with the agent's company and see how they feel about their representation. Do research on the history of the agent's company through the Better Business Bureau. Only take on representation after you've throughly done your homework and you feel comfortable with your agent and his ability to find you roles.
Attend all the auditions your agent can get for you, even if they're not at big budget studios like Warner Brothers. You need experience; big studios won't hire inexperienced actors. Continue to build your portfolio by acting in smaller roles and soaking up as much knowledge about your craft as you can.
Hire an acting coach. Work on acting exercises, improv, voice and dialects. Hone your craft until you feel truly prepared for the chance to be cast in a big budget movie.
Wait. This is the hardest step, as there's never a guarantee that'll ever get to act in a movie produced by Warner Brothers. Thousands upon thousands of people want to act in big budget movies as well. Continue filling up your resume with acting jobs and experience. With a little bit of luck, someday a Warner Brothers part will be yours.