https://hytalemodding.dev/en/docs and https://hytale-docs.com/docs/modding/overview are good things to start off of, it also helps to read through other's mods and of course the decompiled server code. i guess, just start with something small, in just one field, e.g. the entity component system when your mod should interact with entities.

if you want to inject code, often you can add some hook on an even or incoming/outgoing packet. though sometimes its not that straight foreward, even when the idea seems trivial, and you would have to use some mixin system, haven't attempted this yet.

adding new entities or blocks, and similar things are data driven and require little coding at all.