G-Code, often used alongside M-Code, is a type of coding used for machinery. It involves a list of set commands that are ordered with a letter and a number such as G00 or G23, for example. While some places may use a wizard that bypasses the use of this coding, it is always helpful to know the basics of the coding language in case a strange error occurs. Plenty of online references teach this coding.
Read up on the basics of programming to give you a foundation to start from.
Learn about the type of machine you will most likely use. This will tell you the type of commands the machine will use.
Begin memorizing and learning about various G-Code commands from sites such as the ones listed under "Resources." Not all commands listed on any one website may refer to the machine you may be using.
Look into the M-Code as you may have to use it alongside G-Code.