If you really want to learn I highly recommend downloading a client and figuring out how things work, you could use a pre-existing server as a reference. Look into how the client handles packets, how to find packets and also look at how the client reads data from the cache.
Unfortunately, there are very few tutorials around on how to get started and how certain things work, so your only option as of now is to refactor the client and figure out how things work. As I mentioned before, you can use a pre-existing server to see how certain protocols work and to find your way in the client.
I don't have much osrs experience but from what I've seen, rsmod is a good framework. I'm not sure how much actual content it has, but I am fairly sure that all content is in a different package. You could throw out all the "default" content and work your way up from scratch. That would save you from networking and some cache-related things. I still recommend you figure out how they work though.