Learning, test and development while site is live?

So my WordPress website and blog are up and running – very, very cool. (Migrated from wordpress.com. Lost comments, but everything else is finally okay.)

I know there’s a whole world of themes and plug-ins and so much more I could use to improve the appearance and usability of my website. I need to learn this stuff through trial and error.

How do I set up a parallel development and test environment that uses my up-to-date content, but doesn’t mess with my live site until I push the button? Should I do this on my home computer (I can see the eventual upload being a problem), or do most hosting providers (mine is HostGator) provide a sandbox environment with access to my data?

Thanks.