one topic should be enough for this question, please use the search function to get closer insights on this.
Creating dems is addresed by various topics in here so please take your time first to read a bit about it.
If you are searching for a guided tutorial this might help (although it differs from DEM generation in the later steps):
http://sentinel1.s3.amazonaws.com/docs/S1TBX%20TOPSAR%20Interferometry%20with%20Sentinel-1%20Tutorial.pdf
A great example has been showed here:
As we have many topics on that, I am closing this one.