If you want to sell your idea to a producer, you will have to write a synopsis, a logline and a title before worrying about writing the pilot. Since they receive hundreds of ideas, they can't spend too much time on each one of them. They will likely only spend one or two minutes on your idea, so all of those things have to be appealing and making they want more so they will order a pilot script.
- Synopsis: less than 500 words to describe your show. After reading your synopsis, the person has to know: the genre, what the story is about, where is the action taking place, what kind of characters are in the story. They will also know if they want to hear more about it or not. It also help the producer to guess the targeted audience and the necessary budget.
- Logline: kind of a very short synopsis, usually one sentence only. This single line you have to identify your main character(s) without using names (e.g. "Five friends", "An enthusiastic man"), his/her/their goal or problem, his/her/their obstacles. It should also convey the genre of your show. Screenwriting: How to write compelling loglines - D4Darious
- Title: it is the first thing the producer, but also the viewers, will know about your show, so it has to hook your audience. As the logline or the synopsis, it should help people know about the genre of your show (even if some still make the cut only using the name of the main character like Chuck but even there, with a name like that, you know it is going to be funny, but it doesn't tell you about the spy stuff). Bad-named titles can ruin a show (for instance, Selfie had a good cast, was funny enough, but even for a comedy the name was ridiculous and didn't appeal, so the show tanked and was rapidly cancelled). Also, try to keep the title short, it will be easier to remember and thus will help people to talk about it (nobody wants to talk about a show named How to live with my parents (for the rest of my life), even on the internet, where every title is shorten: HTLWMPFTROML is even worse). You should try to make your title catchy, so it'll be easy to remember and make people want to watch (Psych is a good example, it's a comedy about a fake psychic, the pun is nice and since it's a well-known expression, it is simple to recall). You can have a working title at first and chance it later for a more powerful impact on the viewers.
Those are the main things you will need to pitch your idea. Try them on friends and family, even strangers if you want, to ensure that they are good enough. If that is the case, you can pitch them to a producer, and write a pilot depending on his answer. If I were you, I'd still write the pilot even if the producer said no (maybe it wasn't that the idea was bad but that you lack experience to convey it in a short summary), since you could still try to produce it yourself, with some friends at first (a small web-serie), that will help you get some feedbacks.
I also think that making small description of your lead characters will be a good choice. It could help you to write your synopsis and your logline (maybe even the title) but also your episodes, to be sure not to change their behaviour too much at once. And, if your show is made, you will need it to cast the characters.
I am not very sure about all of this, but I had done those research a while back and those were the result. But this also depend where you want to submit your show. If it is for a contest, you'll probably need a logline, a title and the pilot whereas for a producer, you have to sell the idea first and then the pilot.
To write a script, you will need a special format (I recommend that you use a software dedicate to screenwriting (like Celtx, Final Draft, Trelby or Scrinever) so you won't lose time with the format. You have to know that one page equals one minute (so if you want to write a US drama, you'll need around 40 pages per episode and 20 for a comedy).
Most screenplay look like that:
You won't need the "fade in" and the "transition" at first (those are direction for the editor, but they aren't necessary to sell your script). They must be an action line (always written using the present, never past or future) after and before each screen heading. In the action lines, you can capitalised the important words and the first time the name of a protagonist is used.