Send your spec script to a television agent. You must get an agent because networks in general will not accept unsolicited material from writers who do not have representation.
Correspond with television agents until you have developed a successful relationship with one. Choose an agent who not only has the connections that you need but also understands your point of view as a writer and "gets" your work.
Devise a career plan with your agent to identify the Fox TV series to which you will send your submission. If you are submitting a script idea for a sitcom, your spec script should also be in a sitcom format.
Submit your work through your agent. Try to schedule a face-to-face "pitch" meeting with the show's executives.