Come up with a new, inventive, fun, unique or humorous concept. Figure out who your main characters will be. Then write a 1 to 2 sentence description of what the show will be about it. This is called your logline. Then expand your logline into a one page synopsis. This provides a little more information about the main characters, their world and what they all seek. It should also briefly describe what viewers can expect week to week.
Create the pilot's bible. A television bible is a booklet that pertains entirely to one show. It has the logline, synopsis, fleshed out character bios, description of the format and structure of the show, intended audience demographics and at least 10 future episode synopses. This is where you will get to play out your series as a series instead of a single episode. Through the episode synopses we need to see some character changes, plot advancements, twists and turns.
Outline your pilot script. Write out your separate storylines (comedies have at least 2, dramas typically have 3 to 4). Then think about the characters' motivations and personalities. Make sure the storylines are plausible and believable. Intertwine the storylines so they make sense and continue throughout the entire episode. It's important to know that not all storylines have to be resolved at the end of the episode, you can leave one lingering to be resolved in the following episode if you would like.
Open your screenwriting software and open a new document under the television show category. Since this is a pilot you get to choose the structure and format. But know that most comedies have four acts and most dramas have a teaser and five acts or a teaser and six acts. Write your script. Be sure to end each act with a cliffhanger that will draw the audience in and make them sit through the commercials to see more.
Give your script to several people. Have them mark any spelling or grammatical errors you might have missed as it is easy to do after you've been looking at it for weeks. Also, ask for their feedback in regards to the tone, storylines, characters and overall sequencing. Rewrite as needed. It's best if you step away from the material for a week or more as well then come back and read it with a fresh pair of eyes. This might give you a new perspective. Good luck and keep writing.