Well the first step you need to do is find the latitude and longitude limits of the area you're interested in. Easiest way to do that, is open Google Maps (not Earth), navigate to a suitable location right click the mouse and from the contextual menu select "What's here". That will give you the latitude and longitude you require. Rinse and repeat for each boundary of the route.
Depending on what utility you're using to import the DEM you then either need to visit the NASA SRTM site, navigate to the folders and choose the best quality hgt data available. This is supplied in 1 x 1 degree, eg. N45W118 or similar. Alternatively you can use the USGS seamless server to download the data but this is slightly more complex and you need to ensure it's in a format your chosen import programme can read. e.g. Transdem will read Geotiff data but not that in bil format.
