as of today i want to learn more about harajuku!

Well.. i'd first recomment getting to know the main or most popular subcultures and their differences, and later on getting familiar with their attributes!!

J-fashion tends to split into a lot of subcultures, like gyaru splits into styles like kogal, rokku gyaru, mode gyaru, hime gyaru, ganguro gyaru, etc!! or for visual kei it splits into kote kei, oshare kei, eroguro kei, angura kei, etc! That's pretty fun to learn about tbh, because they are quite specific styles and you get to have a specific style name that may perfectly apply to you :3

I feel like getting into the different types is an extra but it's probably important to at least get to identify what are gothic elements and decora elements because you see those a bunch in substyles of other substyles lol

Almost any site online is good for learning! if you're interested into decora i recommend cybr.grl! she has a youtube but she not only talks about decora, she dedicates some of her videos to other popular jfashion styles! and to new arising ones as well :3 

You can learn quite a bit about most jfashion subcultures and such on the Jfashion wiki!!! its a great way to get started and see what styles speak to you most, if you then decide to want to buy clothes its best to buy from Proxy sites! (e.g. buyee, neokyo etc) theres also quite a few fun and interesting documentaries on youtube about any fashion style you can think of too :] 

idk if you still come back to this post, but if you happen to id reccomend whatching interviews that cybr.grl has done with other people in the jfashion community! a lot of those videos tend to kind of talk about the history of the subculturs their apart of in a kind of easy to digest way :>>