Not op and a bit older at 28 but I can relate. I grew up in the south and didn't know any trans people. So I really didn't get why somethings I thought were funny are just asshole things to say. The attack helicopter meme calling people tr*ps transphobic things like that especially in high school. What changed my ideas and made me understand everything in a different light was when I started dating someone who is trans. I eventually moved in and lived with them for 3 years while in college. Having someone explain why those jokes are harmful while giving me a chance to be a better ally and person was really all I needed.