Hi Leo! I have a few thoughts on studying fundamentals.
First I think it might be good to brush up on some observation techniques that will help you with proportions and relationships when you study from life or from reference. It will help streamline the process of learning other fundamentals. Jake Parker's "How to Draw Everything" and Betty Edward's "Drawing with the Right Side of Your Brain" will help you in this area and here are some free, simple, and short videos that also deals with observation techniques.
https://www.ctrlpaint.com/videos/visual-measuring
https://www.ctrlpaint.com/videos/visual-measuring-proportion
https://www.ctrlpaint.com/videos/gesture-drawing-spoons
https://www.ctrlpaint.com/videos/drawing-shape-contour
https://www.ctrlpaint.com/videos/drawing-shape-linear-block-in
https://www.ctrlpaint.com/videos/drawing-shape-negative-space
I also would recommend learning the basics of perspective almost from the get go. Perspective will not only help you with constructing scenes, but it will also help you with making characters, objects, and implementing light and shadow. Learn 1 point and 2 point perspective and 3 point. Learn about ellipses in perspective. Learn about drawing cubes, cylinders, cones, and spheres in perspective. Learn about sculpting techniques for perspective- additive, subtractive, footprint, curves, and cutting holes. Learn about scaling and spacing. After you've spent some time doing basic perspective, learn how to construct drawings out of larger simple masses, following rules of perspective where applicable, and then adding contours and details on top.
Others have given you great resources. I will also recommend the draw-a-box website and Andrew Loomis' "Successful Drawing" I have a few others that I think are good for starting out with fundamentals.
- How to Draw What You See by Rudy De Reyna
- Perspective Made Easy by Ernest Norling
- https://www.ctrlpaint.com/library/. He has free content and paid content. If you look at the free content, look at the free video library section. Sections 2, 3, and 7 may be good places to start for fundamentals.
- Drawsh.com It has good complements to many fundamental concepts.
When I study fundamentals, I try to do 2 things to get a well rounded approach:
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Observe and Copy- From your learning material, life, photographs, and other artwork. This includes many types of exercises. For example, if I'm studying constructive drawing I might: Copy straight from the source material. Look at photographs of different objects, identify their major masses, trace over them and add cross contouring. Look at my couch in real life, sketch it by breaking it down into simple shapes and making sure I'm following the rules of perspective. Taking an illustration I admire, and do a sketch of it, just utilizing simple 3d shapes. The point is to observe and interact with the concepts in many different ways.
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Apply: Make your own drawing or artwork using the concepts you have copied and observed in the first step. This does not have to be a finished illustration and can be a sketch, but the point is to make up your own thing. You can do it without reference or you can use reference as a guide if you want, but do not copy it- change the angle or add different forms, details, lighting, etc to it. So if I'm still studying constructive drawing, after I've drawn my couch from observation, I might draw a different couch-mostly from my imagination, drawing it in a different angle and adding different proportions and details to it.
The good news is that many of the resources listed by people here already have exercises that make you do these things anyway. If you come across a learning resource that does not make you observe and apply the principles being taught, you'll have to be proactive and make up your own exercises or seek them out in other places. Start simple and work toward more complex.
I'll show some of my sketchbook studies I did when I was starting on my fundamentals. They show what I was doing to observe and apply. Looking at other people's sketchbooks really helped me, so I hope mine might be helpful to you.
Some with perspective. Copying source material and observing from reference and life.



Applying the knowledge to made up studies




After I started studying perspective I started studying constructive drawing. Some of it was concurrent with my perspective studies as they go hand in hand.
Copying from learning sources, Observing from photo reference and real life. Starting with big shapes, adding details. Paying attention to perspective. Starting with simple objects, going to more complex.



Applying the knowledge from my observational drawings. I constructed the basic shapes without reference, and only looked at reference to help me with aspects of the details.

Working to more complex, I constructed still life arrangements from reference.

I applied the knowledge from the last study and made up a still life with no reference.

Anyway, good luck! It can be hard to know what to study and how study effectively. I hope you find something that works for you!