Drupal 7 DEV/TEST site Question

Hey all,

Can someone point me in the direction of the best place to research establishing a new dev site for an existing live Drupal 7 site?

I found https://www.drupal.org/creating-drupal-test-sites but it has not been updated for awhile now.

My situation is that I have a site that was built using Drupal 7, but with no dev site back in the day.
Now I need to do a major overhaul style wise: install a new theme, make responsive etc. I am learning more and more about Drupal daily through tools like BuildAModule but need my own TEST version of the current live site before making any changes live.

Thanks in advance...