Substantive knowledge is knowledge that is foundational to all other knowledge. It is knowledge necessary to understanding the relationships between all aspects of the world from the physical to the intellectual, from how machines work to how societies work. These are the subjects to include in your pursuit of knowledge.
Your own language, since most of your learning and thinking will be done in your own language knowing how to use your language correctly is required for the most effective learning and thinking. Knowing your own language will also give you self-assurance when dealing with others because you will be confident in your ability to communicate. Learning your own language means developing as large a vocabulary as possible, having a thorough command of its grammar and syntax, understanding its best usage, being familiar with its idioms, and a command of its rhetoric.
All learning and thinking is by means of language and both how much and how well you can learn or think is determined by how well you use your language.
Other languages, especially with roots other than your own. It is not necessary, but advantageous to have some familiarity with Latin, Greek, any of the Romance languages, and English.
All of the hard sciences: physics, chemistry, biology and all their sub-categories. You should have a minimum understanding of the basic principles in each of these as a minimum to what is considered "high-school" level (not something called, "general science,"). The more you learn in any of these sciences the better.
Some sub-categories of sciences you ought to have some familiarity with include, mechanics, dynamics, electricity, the periodic table, organic chemistry, genetics, taxonomy, anatomy, and medicine.
Geography: both physical and cultural geography are necessary to understanding the world you live in. Almost no news today is possible to comprehend without knowing where in the world every country is and its relationships to every other country. Understanding where the countries of the world are, what their natural resources are, what their culture and language is like, how they relate to other countries historically, culturally, and economically are necessary for understanding the world one lives in and even for one's enjoyment of it.
Technologies: including mechanics, electrical engineering, electronics, digital electronics, analog and digital transmission, information technology, chemical technologies, electro-chemical technologies, medical technologies, pharmacy, and many more applications of the sciences to everything from research to industry. One should have at least a general knowledge of as many technologies as possible.
Mathematics: from arithmetic to algebra and trigonometry, as well as plane and solid geometry. If you have the aptitude and interest for it, go as far as you can in the Calculus. Theoretical math, number theory, and "pure" mathematics can very interesting, but are not very useful and math theory is fraught with nonsense.
History: especially a general understanding of the history of one's own country, then of the world from pre-history to the present. Almost everything else you will learn will be influenced by history and be related to it. History, like geography, will expand your entire view of life and reality.
Literature: of all kinds is valuable. (See Reading below) Here it is literature in the artistic sense that is meant, like fiction, plays, and poetry.
Graphic Arts: while some so-called art appreciation subjects may be of value, it is the nature of graphic arts themselves that are most important. How are they created, what is the nature of color, shapes, perspective, shadows and light. What are the mediums from drawings to paintings to digital graphics. Such knowledge provides an enormously enlightening understanding of how and why the world and things look as they do and increases your capacity for enjoying the beauty of existence.
Performance Arts: music, dance, theater, opera, radio, movie, and television performance are things one should have some knowledge of because they are so much a part of modern life, as well as history, and one's own understanding of those aspects of the world.
Practical Knowledge: is knowledge that is most useful. All knowledge is useful for thinking and expanding one's scope of understanding. Practical knowledge is the kind of knowledge one uses every day for making decisions and performing life's chores. One may study electricity and understand the basic principles of current, voltage, resistance, inductance, and capacitance, which are necessary to understanding how fuses and circuit breakers work and how a house is wired. To use that knowledge one needs to be familiar with the color code of AC wiring, how to change a fuse or reset a circuit breaker, how to connect a ceiling light or fan, as well as knowing how to use the materials and tools used in AC wiring, such as electrical wire and cable, switches, plugs, connectors, electrical boxes, wire connectors, wire cutters, and wire strippers.
These are a minimum of the kinds of practical knowledge one might have: plumbing, cooking, sewing and clothing repair, cleaning (everything), basic carpentry and building repair and maintenance (painting, window repair), gardening, basic finance (balancing a checkbook), basic auto and machine (lawn mowers, snow blowers, generators) repair and upkeep, how to use a computer, touch typing, reading and understanding maps, schematics, and blue-prints, foraging, writing (letters, reports, proposals), public speaking, and knowing what all the tools are required for all of these and how to use them. The list is intentionally broad and only a suggestion of the many practical kinds of knowledge that increase one's competence to deal with any of life's situations.
Philosophy: Philosophy is the most important of all knowledge because it is the foundation on which all knowledge is based. Unfortunately, almost all that is called philosophy today belongs to the category of, "false knowledge," discussed below. Nevertheless, one must be familiar with what philosophy is.
The field of philosophy consists of seven categories: metaphysics, ontology, philosophical psychology, epistemology, ethics, politics, and aesthetics. Metaphysics is the study of the nature of reality and existence, ontology, a sub-category of metaphysics, is the study of the nature of material existence, philosophical psychology is the study of the nature of human consciousness, i.e. the mind, epistemology is the study of the nature of knowledge, ethics is the study of those principles on which right human choice and action must be based, politics is the study of ethical relationships between individuals in society, and aesthetics is the study of the nature of beauty.
Any philosophy studied in an academic setting will mostly be the "history of philosophy," or the philosophy of some individual or a particular, "school," of philosophy, which is almost universally mistaken. Nevertheless, it is good to have some knowledge of that history and those wrong ideas in philosophy because they influence so much that is wrong in all areas of modern life. It is not necessary to take any philosophy courses to gain that knowledge however, because there are some excellent books and online sources.
The Moral Individual provides several articles and a book dealing with most of the categories of philosophy—true philosophy—with the added advantage that they are free and you may ask questions and get clarifications directly from the author.
Semi-science and Pseudo-science:
There are many subjects which are called science because they use some of the methods of the physical sciences and much of the same language. I call them semi-science. It doesn't matter what they are called but they do not have the same kind of certainty as the principles of the physical sciences and are not as useful. Most of these are observational disciplines, like astronomy, taxonomy, geology, oceanography, archaeology, paleontology, and meteorology. There are two semi-sciences which are studies of origins: evolution and cosmology, which are both hypothetical or conjectural. These are all legitimate disciplines providing some real knowledge, except for the last two which one ought to know about, but not necessarily accept.
Phenology, anthropology, psychology, sociology, economics, and environmentalism, are pseudo-sciences which means they are not sciences at all. It is good to know about them even though almost everything in any of those fields is either untrue or useless.