You know, while Lee's advice is good and practical, I have a different take on plug-ins. I had been an artist for 35 years before ever picking up a mouse, in 1998. Within a year I had settled on Photoshop as my favorite application and always the last satge of all my images prior to printing. Anyway, when I began the computer and PS, I was already accustomed to visualizing and immediately being able to create. Not so with PS. It is slow going to learn computers and have at Photoshop as your first major app. Plug-ins, I found, gave me a way to fashion images which were sufficiently appealing to keep my interest high. As a result they helped immensely.
You see, unlike most people who think they have to learn how before doing, I just have at things and do them. Of course learning is a major component of the process. You have to learn in every piece or you always stay as awful as when you were a beginner, but doing is the heart of it. As I've kept learning, the third party plugins may not be used as often any more, yet I still have great affection for the wow factor which they can stimulate in the beginner. Thus I'd advise, if you can afford plug-ins and get a kick out of them...have at 'em!