How should I learn JavaScript?

There are a plethora of resources on the web for learning JavaScript. For this course, we would recommend you do the following:

Work your way through the extensive notes that follow this page, which have been designed for people who have done our prerequisite modules.
Supplement this with resources such as CodeAcademy. There are also good tutorials on w3Schools.
Check out Code Combat. I have set up a virtual classroom for 2020 web-based GIS students. This is a fun, interactive way to learn JavaScript. Let me know how you get on!

HTML & CSS are also worth exploring for improving the look and feel of your website, and both CodeAcademy and w3Schools have sections on these.