Put a volt meter or simple test light on the hot post (the post, not the cable end or nut) on the starter. If it lights up when you go all the way to start on the key switch then the starter is bad.
It may be a simple fix internally or a replacement starter, either way the starter will need to be apart to find out whats wrong with it.
It may be a simple fix internally or a replacement starter, either way the starter will need to be apart to find out whats wrong with it.