Know your body, learn to know your body's normal Function, tolerance, differences, changes, etc. Whenever you find that Something doesn't feel or seem right, go to the hospital or healthcare centre for Proffessional opinion. Medicine is a whole Package ,you can't simply learn Anatomy or pathology then all over sudden you will know if you need to see the Doctor or not. Unless you have 6 free years full time so you can learn it enough to safely make such a decision. You can see your Primary care Physician for healthcare counselling or advice you don't have to be sick to inquire about your Health. I personally Encourage it.
Hi shelby , your post should have said you want to learn about physical injuries as you have replied.when you say "i want to learn about medical things so i can know when to go to a doctor " thats a whole different idea . That being said you can start on webMD and Healthline ,both trusted sources on all things medical and language is simplified too. Goodluck:)
Learn about anatomy and it’s functions. That’s a good first place to start.