You tube university. Lots of freecad videos. just be sure you are watching videos for the version you are using. A lot has changed over the last year or so.
The first thing to consider is the construction techniques avaliable to you and their limitations. For example, a sheet bending product workflow will be different from a lathe product workflow. Subtractive methods such as milling or latheing have the least manifold space freedom. If you are using a 3d printer without multimaterial capabilities, keep in mind the supports and the bridging. Injection molding must have venting and spures. If the 3d printer has multimaterial capabilities, you have the most manifold space freedom if one of the materials is PVA filament.
I would at least draw the shape of what you want on paper before you draw it on the computer. Also list any contraints the part needs. Your ultimate constaint is from your tooling!
CAD work is all about parametric modeling where you define features. A feature is anything that adds or removes material. Your feature will almost always be defined by a sketch for two deminsions. The third demision is defined by a feature tool.
The order of how features are defined is important, because elements of previous features can be built on top of it. This can be a blessing or a curse if a parameter needs to be changed. If you mess up, you "roll up" the feature "timeline" and rebuild from the first feature to your last. Its not uncommon to start from scratch again when feature relationships become more clear on which feature should parent another.
Always save filleting for last. They are used for strength for convex features and are critical for tool paths in concave features.
FreeCAD is a difficult package, but if you master it, SOLIDWORKS and Autodesk will be a joke because of how easy they are.
Eventually, you will be able to define parameters for features in a spreadsheet. This will allow to create "procedural" parts quickly. For example, I used this for someone who needed custom gem indexing gears. Generally, you can assign variable names to feature parameters.
Also get yourself a decent pair of calipers and maybe the Machinery's Handbook.
I taught myself by watching JewTube video tutorials for my flavor of software. (FreeCAD) Having two screens, one for the video and one for the software was useful but a single large-ish screen will do also. I had some background in technical drawing and 2D CAD but not a lot. It's not a requirement but it helps to know basic rules of geometry.
I only model technical things, no minifigs or artsy stuff. Recommend you start out with low complexity models to get the hang of things and keep frustration to a minimum. It was immensely rewarding btw, to see an idea that only existed in my head, pop out of the printer, so keep at it.
I started out by importing geometry I had prepared in 2D CAD and going from there. I wouldn't do that today, the design tools for a sketch are all there in the 3D software but the design paradigm is somewhat different.
In 2D CAD you make a 2D representation of a thing and usually someone else will have to actually make it. This means you must communicate all dimensions and parameters through your drawing. It needs to be comprehensive.
In 3D CAD, especially if you only model objects for your own use, you can get away with a less strict approach although it'll pay in the long run if your objects are created such that you can open them a year later and still make sense of what you were doing. In standard 3D CAD software you'll have the opportunity to name your sketches and 3D operations. I use this to have some mnemonic naming like: "Hole d4.5" "Pipe cyl D20 d15 h100"
Got any concrete projects you want to do? I'm sure others on here would help out if you get stuck.
