Well I had 15 years of prior programming experience which helped.
The rest was learning the object model Minecraft uses. In the end, it depends on how best you learn new things.
If it's a simple mod, then it isn't too involved, as the complexity of the mod goes up, so do the number of things you need to take into account.
Downloading the forge source, eclipse, and an opensource mod. Go through the code and start picking and prodding at it.
