Typing c when the staging area is unused is a special situation. Normally, the next commit would be empty, but you can configure Magit to do something more useful by customizing the magit-commit-all-when-nothing-staged variable. One choice is to instruct the subsequent C-c C-c to commit all changes. Another choice is stage everything at the time of hitting c.
You can type C-c C-a in the buffer with the change description to toggle a flag that determines whether the next commit will amend the current commit in HEAD.
なにもない状態で
c→C-c C-a
