The Basics
Importing Libraries
Traditional Variables
Magic Variables
Global Variables
Strings
Declared Metadata
Property Modifier
Key Modifier
Type Casting
If & Else Statments
Repeat & Repeat Each
Functions & Macros
Menus
Jelly is an extremely simple language, it pulls from Swift and Javascript for most of it's syntax. However some parts of the language like Metadata are Jelly specific. If you already know one of these languages, learning Jelly should not be very hard. If you do not know either of these languages, the language guide will walk you though everything you need to know.
I would recommend getting started by reading Writing Your First Script in the Getting Started section. After you have written a simple script, look at this section for topics you do not understand or want to understand more about.