Hi Barclay and welcome.
To hack you have to understand the basics of networking. In any network there have to be
3 basic components; a client, a server and a protocol. For example you asked a question here, so you are the client. I am answering you (giving you a service), so I am the server. The protocol we are using to communicate is the English language.
The art of hacking means that you tweak these 3 components to get the maximum out of them, and so that they do what you want them to do (which is not necessarily what they were originally meant to do). If you (the client) were to ask me different questions or to change the syntax of your questions, you will get different answers from me (the server).
You can't tweak something until you understand how it's supposed to work in the first place. Different types of servers may react differently to the same request (different OS's or different applications). For example you asked one question yet you get 2 different answers - one from Don and one from me.
Also you have to understand how to troubleshoot - if you ask me a question in English and I answer you in another language, or don't answer at all, you must figure out if this is a client, server or protocol issue.
In modern networks, the most popular protocol is TCP/IP. Don has posted a link for more info. I also recommend that you read through this sample chapter
from Michael Greggs' CEH study guide.