How to commit a change in git when git commit opens Nano?

After typing your commit message, try:

• ctrl + o
• enter
• ctrl + x

That screen is just a text editor and those options at the bottom, represent commands, tipically its the ctrl key + the letter for the command.

To make the commit you should write your commit message, then press ctrl+o to write your message, and then ctrl+x to exit that screen.

To avoid that screen you could do something like git commit -m 'your commit message', the -m indicates that your commit message goes in the command.