To be honest, the best way to start is simply to start. Take baby steps and focus yourself. You've mentioned doing single cavity molds to begin and that is probably the best idea you could work with. Once you are familiar with how the plastic handles and you learn what some of the little nuances of plastic are you'll get to where you'll want to advance a bit. Work at a level you are comfortable with....the only one you need to impress is yourself and if you have some duds come along, chalk it up to experience. Learn from the mistakes and forge ahead.