There are two common reasons for having ghost signals with a metal detector. The first being that you could be wearing steel toe boots and are getting too close to your foot. The second being that the object is so small that you dig it up without realizing it.
For the second reason, people tend to use a pinpointer. A small wand-like device that can help you find tiny metal objects.
When you are metal detecting, what you should be looking for is a nice steady tone. That tone should be the same if you pass over the same spot more than once.
Once you get the hang of it, you will begin to notice the tone differences between coins, pulltabs, nails, and other objects. But there is nothing wrong with digging everything up.