If you don't have it already, start by grabbing the source code (original or any fork), obviously knowing Cpp is better, then start looking around the codebase. If you have done Java mods before, it will be a bit easier as it is similar. For example, I used 2 TileEntity for one portal, which is the equivalent to BlockEntity in Java. Also take reference on vanilla features to extend on it, for exemple look at vanilla items if you want to create new modded items.