After making changes to your source code, you should stage those changes with Git before you can commit them.
For example, if you change README.md and program.py:
git add README.md program.py
This tells git that you want to add the files to the next commit you do.
Then, commit your changes with
git commit
Note that this will open a text editor, which is often vim. If you are not familiar with vim, you might want to know that you can press i to go into insert mode, write your commit message, then press Esc and :wq to save and quit. To avoid opening the text editor, simply include the -m flag with your message
git commit -m "Commit message here"
Commit messages often follow some specific formatting rules, see Good commit messages for more information.
If you have changed a lot of files in the directory, rather than listing each one of them, you could use:
git add --all # equivalent to "git add -a"
Or to add all changes, not including files that have been deleted, from the top-level directory and subdirectories:
git add .
Or to only add files which are currently tracked (“update”):
git add -u
If desired, review the staged changes:
git status # display a list of changed files
git diff --cached # shows staged changes inside staged files
Finally, commit the changes: