So far, we assumed you already have a Debian™ package to start with, but what if you want to start a new package? First, create an empty repository for your new package. We assume you want to package a software called named mypkg:

 git init --initial-branch=debian/latest mypkg
 cd mypkg

You can then import the upstream sources and add the Debian™ files (e.g. via dh_make):

 gbp import-orig --pristine-tar --debian-branch=debian/latest --upstream-branch=upstream/latest https://example.com/releases/mypkg-0.1.tar.gz
 dh_make

gbp will create the needed branches, detect the upstream package name and version automatically, ask for confirmation (unless you pass --no-interactive) and create a git tag for the imported sources. If you don't want to use pristine-tar you can just drop it from the above invocation.