Long answer:
First of all, you need to know how to code in JavaScript language. It is recommended that you learn about HTML and CSS too. The site w3schools.com has good info about this three scripting languages: JS: http://www.w3schools.com/js/default.asp HTML: http://www.w3schools.com/html/default.asp CSS: http://www.w3schools.com/css/default.asp
Then, you need to know some rules to write userscripts for Greasemonkey or Tampermonkey: GM: https://wiki.greasespot.net/Greasemonkey_Manual TM: https://tampermonkey.net/documentation.php
For some technical questions, you can find related answers in stackoverflow.com
Short answer:
Both answers are very good and I would recommend them.
How to make a Script
how to make a script
i want to know