1) load the shapefile over the raster. (the shapefile and the raster image must be in the same coordinate sistem).
2) select the shapefile, go to copy and then paste > you will make an AOI from the shapefile boundary
3) select the AOI (it will turn to yellow colour)
4) go to Raster tab > Subset & Chip > Create Subset Image. (to clip the raster using the AOI you must press the "AOI" button > from View).
5) insert the name of your clipped raster and press OK > now you have a subset raster using the shape file (AOI) boundary
Finish.
IF YOU HAVE PROBLEMS and you don't understand my steps, go and search on Google or go to ERDAS forum.