This is such a big topic that people might be a little reluctant to start with an answer.
It seems that you want to specialize in designing objects for printing - as opposed to character or environment creation, animation, special effects or general purpose 3d modelling. Keeping this in mind, you shoud start by acquiring a copy of Blender (which is free and does export models in STL file format for printing). There are many free tutorials for Blender that will equip you with the fundamentals. So, start with those. Some here might suggest that you start with a program called "Rhino" or with Autocad, as these are specialized for computer-aided-drawing and in wide use by engineers. But, since you seem to have a primary interest in printing objects, rather than designing buildings or designing sail boats, Blender probably is the better choice for you to begin.
I am not a Blender user, so I cannot recommend any particular tutorials. But, I will offer that commercial tutorials (meaning that you pay for them) often can get you started more quickly than the free tutorials. However, this is not always true, as there are many professional-grade, high quality free tutorials for every modelling application. Perhaps some of the other Blender experts here can make good suggestions as to beginning level tutorials for Blender.
Anyone.....?
The basic concepts of 3D model-building are universal and do not depend upon the application used. You can find these basic concepts in the first chapters of almost any book on 3D modelling in a good bookstore, or you could try some of the Autodesk Introduction tutorials, as these are in both written and video form, and available in a number of languages. Here is one, for example. (https://area.autodesk.com/tutorials/intro-to-maya-lesson-1-basic-skills/)
It is preliminary to deal with the question of "good people to follow and to learn from" as you first need to focus on mastering the basics of model-building.
So that you know, there a quite a few things to learn all at once, when you start. So, you must expect to need to absorb a lot of technical concepts and skills that won't seem entirely related to model-building. This is because you need to learn how to create an object, view and manipulate it in 3d space (learning the coordinate systems), but also how to render (make a picture of your object) using a "rendering engine" that is a separate component of the application with its own rules and procedures, learn how to "light" the object so that you can see it in the render (lighting is a separate subject), and possible even how to use a camera (many 3d programs allow you or require you to "see" your 3d object through a camera), and, of course, camera properties also are their own subject.
But, the good news is that after you have acquired some basic knowledge and skills for viewing and manipulating a simple object, lighting it, applying some simple materials to it (another subject that again has its own properties and knowledges), and making renders of it, you then will be free to focus on object creation.
Almost all objects in the 3D world are made with just a few primative polygons or "NURBS" curves and surfaces. So, the next set of steps focus on becoming familiar with these primatives, and then learning how to change their shapes and assemble them in various ways to build up a model. You will want to focus on "polygonal" model-building for a long time. (You can take a side trip and learn what NURBS modelling is, but this should not become your focus or model-building style. At least, not in the beginning.) Similarly, you can take another side trip and investigate "character-modelling." But, even though your business may make use of a lot of pre-built 3d characters, you should not spend much time on this, instead focusing on the fundamentals of polygonal model-building. So, look for beginning-level or "introductory" tutorials on polygonal modelling.
And then, when you are comfortable with this, and can build complex objects for printing, come back here and ask more questions.
One other question you asked, I should like to give an answer. You asked about related skills and things. For someone who needs to focus on polygonal model-building for printing, and is just getting started, I would recommend purchase of one specialised piece of software. This is called "Balancer", and I would recommend the "n-Pro" version. This is a tool that "reduces" the number of polygons in a model to help reduce its overall file size, but does so in a way that is particularly designed for 3D printing. It can help you to maintain the depth and size of inner spaces needed for printing. When you are first learning, it is easy to make models that have too many faces to print well. So, this is a tool that is auxillary to a model-building program such as Blender.
So, this is just a little "start information."
