There are at least 2 things in medicine that represent true foundations, in that knowledge of same allows you to have an intelligent conversation without becoming totally lost. Those two things are: 1) anatomy, and 2) physiology. Parenthetically, There is also a particular skill that is fundamental, that of taking a history & physical exam; this will be covered later.
Anatomy is a basic description of where things are in the human body, and how they connect. Physiology is the description of how the various organ systems do their job, whether that job is contraction of muscle fibers to produce a heartbeat, or how your liver detoxifies that beer you sipped just before sitting down to this post.
Unfortunately these particular topics favor, and at a certain high degree of mastery require, brute force memorization. I am good at that, but never found it particularly fun. We will attempt to ease some of the pain by incorporating the relevant anatomy and physiology as running themes, into a larger discussion in which you will see some connection between the book and the real world.
To start out, we will refer to the Kaplan Anatomy Coloring Book, available here from Archive.org, supplemented with the 2001 edition of the Special Operations Forces Handbook (link is a pdf) for anatomy, as well as a 2008 edition of Hurst’s Pathophysiology review; hard copy here. These are both of sufficient complexity to enable you to get the job done.
As you review these texts you will naturally pick up a great deal of the general vocabulary; however, should you get lost the Medical Dictionary is a good online resource.
And speaking of good online resources, for our purposes Medscape has a great reference section, allowing you to look up medications, diseases, procedures, anatomical descriptions, etc. There are others, some I even think are better, but they are most emphatically not free.