Writing a script for a killer TV show pilot is not easy. You need to start with the basics
1) Chose a subject and research it well. Even if you are writing a script for a TV detective series, you need to find a new twist, something that has not been done before. For that you need to build credible characters, a plot with a twist and a gripping story line that flows through the script.
2) Know how to write a script. If you have not already taken a course in scriptwriting, then you should consider it. Producers do not like scripts that are not properly presented. If you want some examples of scripts then the www.bbc.co.uk is one place to go. On their website they have examples that you can study. They also have templates that you can download and use.
3) Plan the series. If it is a pilot script, then before commencing the pilot you need to have threads that will run through the whole series, and indeed the second or third series if you are fortunate enough to be taken on. Inspector Morse, a series which ran in the UK ran for well over ten series. Often there were references to things that had happened in the personal lives of the characters in previous years.
4) Do not be afraid to co-write. Yes, we all want it to be totally our own work, but sometimes it can be easier if you ar writing with someone else. Sure you will lose half of the payment, but then half a cake! In addition this process is like to lead to much more variety in both the characters, plot and locations.
5) Research is essential. For any writing, research is vital. Nothing worse that submitting a script to then have it refused because is technically incorrect or something that you have asked characters to do is physically impossible, unless it sci-fi of course. Make sure that it is believible.
Finally, if you can get a good agent then do so. Good luck