I am a big fan of ‘learning by doing’, which in an ERP environment could be quite dangerous. 🙂 You want to start with learning the basics through some training (there are many partners that provides this) and get the Microsoft certifications (good way of setting an initial goal I think). Then continue building your experience with tasks that are low risk, like creating reports, adding fields, etc. Working along a more experienced developer is a good way of learning what to do and what to not do.