Buying or building a cutting tool is only the start of many years of spending sometimes substantial amount of cash.
From my experience the best way to learn anything is to have a project, that way you can focus on getting one thing right at a time. Set yourself a task, maybe you are into clocks, loco's, boats, aircraft, what ever. I think its time to decide what you are going to make on the machine, then gear yourself up to achieve this goal.
Think about it this way :-
What am I going to make?
What materials will I be cutting?
What tools will I need?
Do I need 2D, or 3D code generation?
How am I going to hold the work pieces?
Do I need special measuring tools?
Once you know the answers to these questions, the next list of questions will be automatic and a path will develop
At the end of the day the G-code part is the last bit you need to worry about, and we can help you with that bit too.
Graham.