Hi, while there are many guides to using Git, Subversion and the many other tools, what it boils down to is to tell the tool when you have made a "significant" change, preferably one that is in self-contained (i.e. not part-baked) and give that change a name. The tool keeps track of the changes, and lets you rewind the files to a past state, view the sequence of changes made so far, and display what actually changed at what point.
For Git, using the basic Terminal tools (many others available):
# create the files git needs to track this folder and its subfolders:
# add all files in this directory to the list of those being tracked:
git add .
# and save this as the initial state:
git commit -m'new project'
... make changes in kicad and save them, then:
will tell you how the files actually changed as a result of the edits.
git add ... files... main.sch
git commit -m'Added new power system'
To explore the history of changes, use: