The program is very powerful and has a lot more tools in it than you'll need. The gist is you get the DEM file from, I'll assume here, the NSGS National Server.
The National Map - Advanced Viewer
You scroll zoom, scroll, etc. to the area you want.
Pick the data to download. IMG, etc. This is under the 3DEP or something like that, I'm going on memory here.
You can also download current imaging here and historic maps going back to the 70s and 80s. Choose GEO PDF and then convert the files within TransDEM
For historical maps go here:
USGS Historical Topographic Map Explorer (arcgis.com)
To be able to fix-up and use the GEO PDF files, you will need to install the GDAL application. The link is provided, I think in TransDEM documentation, or on the website. You install this separately and it's called up by TransDEM to convert the files to work properly after TransDEM trims them.
Anyway, before I get too far ahead and confuse you.
I highly recommend going through the tutorials available in the documentation. PM me if you need help. I also put together a couple of quick tutorials that I can send you. Unfortunately, the NSGS website changes often, so it's difficult to keep the docs fully up-to-date.