When you run the command
crontab -e it typically defaults to the
vim editors. If you type the command Shift+Z+Z you can save any changes in this editor and exit.
To add entries to your crontab using this method you'll need to learn how to use this editor more extensively, which is beyond the scope of this question, and should be easy to find many tutorials on the internet.
vim is too much of a learning curve you can instruct
crontab to use a different editor. Another console based editor that's easier for new people to Linux is
nano, it's typically installed on most distros that I'm familiar with.
$ EDITOR=nano crontab -e
NOTE: To use
nano's menu all the carets (aka ^X) commands at the bottom require the use of the Ctrl key. So to exit, Ctrl+X, for example.
You can of course use any editor here. A easy GUI based editor, if you're using a GNOME based desktop, would be
$ EDITOR=gedit crontab -e
This last one might be a challenge to use, for a different set of reasons, if your primary desktop is being run by a user other than
root, which it likely is, so I would go with
nano for starters.