Step 0. Set your goals
Where you start depends on what you aim to achieve.
Step 1. Learning from experience
I would recommend starting with something you can easily approach, like logo design or simple print jobs, flyers, etc. Watching tutorials on Youtube will teach you how to use this or that software, but that doesn't make you a designer.
Getting a job in your area, not remote, not freelance, but an actual 9 to 5 job will increase your learning curve as you will be faced with pressing situations when you will have to deliver quickly, which tends to favour learning. Try going for an intern position with an agency and learn from their prepress department. DTP/prepress guys know a lot of technical things a designer doesn't necessarily need to know. Go bowling with these guys.
Taking a class also counts as good experience. Online classes tend to be similar to Youtube tutorials, so better look for actual paid courses which can span multiple days or weeks and you learn practical stuff.
Client relations is another skill you need to build. Presenting your work, revising your work, invoicing, accounting. These have their own jargon.
Then, graphic design is a wide term and there are quite a few sub-fields with their own technical issues and jargon.
Do you want to be a print designer and mainly work with printables (as your question might suggest), eg. brochures, magazines, books, reports, flyers, leaflets, anything on paper basicly. Doing some logos every now and then could also require print knowledge.
Then there are typeface designers, animation designers, illustrators, etc. Each of these jobs require different skill sets.
Will you want to cover all these? That could be overwhelming to start with. Back to Step 0.
Step 2. Learning from resources
Obviously there are books which can teach you the history, constructing and deconstructing designs, grids, typesetting, packaging, prepress, automation, the golden ratio, you name it. Then there's blogs and Youtube tutorials and all that free-for-all stuff. Even professionals will spend time on Youtube, since you can never really know everything. You learn new skills with each new experience (read: real-life project). Resource learning will not get you too far without being involved in actual projects, as many as you can. Back to Step 1.