I was able to create ~/doc, but I don't know how to add the file.txt directory to it.

If you're working from the command line (shell),

• The command to create the directory is mkdir
• The command to create the file is touch (or any editor)

I should be in the home directory ~ right after I log in, so

$mkdir doc
$ touch doc/file.txt# 0 byte file created
$ You can start the vidoc/file.txt# editor and write whatever you want.

You may want to move the current directory before you operate it

$mkdir doc
$ cd doc
$ file.txt

vi (or any other editor) Find out how to use it yourself (you can ask a different question)