Minecraft with the libraries its using is too big for a beginner to really work with. A good amount of modding is researching the existing implementation to find the most suitable avenue to alter it. FML/Forge certainly doesn't help by using rather niche techniques like reflection and byte code manipulation even for basic tasks, making it difficult to e.g. get an accurate idea of what happens before your code even runs. Besides that the code quality is lacking.
I highly recommend to start with basic standalone software as taught by your average university programming course. Mathematical problems and some basic data shuffling are ideal to learn how to express yourself in code, the development environment, debugging and the most important parts of the standard library.
Scala should be avoided, it's more of a play ground and too easy to misuse.
Agreed. Java is the obstacle, not modding usually. And as for Scala, it is great, but the fancy neatness can be deceiving. You need to know your shit.