I use eclipse myself. Get a firm understanding of the current modding system. Do expect yourself to master it quickly. You will most likely get really frustrated having ideas but not know how to implement them. I have a library Lazy Library here that creates a system of automated items and blocks. Feel free to look at the code to see how I do things. Developing addon's is a good place to start. Learn about what the original mod is doing by looking at its code. Get familiar with debugging tools. Most of it is experience and you'll get better gradually.