It depends so much on how clear the dancer is in front of the background. The usual ways of making it happens 'automatically' are by luminance (brightness) keying or Chroma (colour) keying. So a bright object against a dark background can be isolated as can a dark object against a bright background. Colour keying means the green or blue screen background. If you have a dancer against a random background, then it's much much more difficult, and although there are a few (quite expensive) plug-ins that can help - in most cases the process means you draw around the dancer, and delete the background, frame by frame. Some of the plug-ins let you set one frame like this, then they attempt to track through frame by frame - and if they get a little lost, you adjust the mask and keep going. Adobe has a nice way of doing this in a simplish manner - but it still takes time, and the result depends so much on how good your original footage is. Some backgrounds take almost forever to separate, others are nicer to the auto systems.
What editing software do you have - this is important as it gives us a guide as to what may work best for you.