how to get slope from dem?

I use software ERDAS to extract slope from DEM , but met a problem,
as the dem image's domain is my study area, and slope extracted always
relates to surrounding pixels, so the border of slope image is very
white in gray image,is it right? how to deal with it?thank you.

Not sure what you mean but the slope at the edges is almost always
incorrect and should be masked out afterward. If you must have the
slope model extending to your cureent limit then the best thing to do
is to obtain more DEM. If that isn't an option you could try
extrapolating out, using kriging but his will only be a calculated
guess and not reliable as a model on which to base further study.