I worked through http://gitref.org/
To clone wakarimasu (a public repo)
git clone https://github.com/whitespace-software/wakarimasu
To clone gala (a public repo)
git clone https://github.com/whitespace-software/gala will ask for credentials
To initialise a Git repo in current directory
git init
To see the current status
git status -s
To add some files, including modified ones that have been added before
git add file1 ... fileN
git add . ... adds all files in this folder
To see differences
git diff
git diff --stat
To set credentials for a commit
git config --global user.name 'AJC'
git config --global user.email 'jonathan@bitwix.com'
git config --global user.name to see the current value of user.name
To commit the changes, adding -a to do the add as well
git commit -m "more lines in HTML"
git commit -am "more lines in HTML"
To see the details of the GitHub repo
git remote ... blank
git remote -v ... blank
git remote add github https://github.com/whitespace-software/HowToUseGit
git remote ... now lists github
git remote remove github ... to remove that name

It may not be possible to create new GitHub repos from the command-line without using the website https://help.github.com/articles/adding-an-existing-project-to-github-using-the-command-line/ is a mixture of website and command line

To push up to the GitHub repo (as long as the repo already exists)
git push github master ... will ask for credentials