You're here because you heard that this would a good idea. There are going to be many ways to ensure a directory is 'secure', but that will depend on your requirements.
I will assume that you are the only person that will need access to said directory. I also will not try to explain unix permissions to you. If you want to know more I suggest you read the link at the top of this page. Follow along below to create a directory that only you will be able to access.
Given a directory foo we can do the following:
chown $USER:$USER foo
$USER will translate to your username, but we recommend you actually type out your username. This will set the user and the group to $USER.
Now make sure that only the user field can has access to the directory.
chmod 700 foo
I got your back.
chmod -R go-rwx foo -R: recursive. g: select group permission o: select other permission -: remove r: read permission w: write permission x: execute permission
We are asking chmod to remove read, write, and execute permissions on the group and other permission fields.