The procedure for opening the S-1 zipped products in the S-1 Toolbox is the following:
1. Open S-1 toolbox
2. In the file-menu, select
Import Raster Data/SAR Data/Sentinel-1 and import the product
3. The product appears in the Products View window. Click open the bands in the tree to display amplitude and intensity bands
In order to subset one should first display a band, which can be done by double-clicking on a band.
Note that the image can be flipped due to the peculiarities of SAR imaging (if you want to fix this you should do
SAR Processing/Geometric/Ellipsoid Correction/Average-Height Range-Doppler, but it's advisable that you subset first due to the huge size of the imagery. Amplitude looks better than intensity, or you might want to experiment with intensity in decibel-scale which you can create in the Product View by right clicking on an intensity-band and selecting Linear to/from dB.
Once you are seeing the band you want to use for subsetting/cropping you should zoom to the view you want, right-click on the image itself and select
Spatial Subset From View, or if you want just a simple image-file instead of a satellite-product you can also select
Export View as Image.
If you keep working with the subset in the S-1 toolbox you can do many things like experimenting with RGB-views, multilooking in order to reduce both resolution and speckle-noise, correct the scene geometrically into the map-coordinates of your choice. If you have a DEM you can orthorectify the scene and get rid of foreshortening that makes mountains look weird in SAR imaging. These images are not photographs like optical data so the learning-curve is steeper, but also rewarding