I believe experimenting with your blog is crucial to success, but you don’t want to experiment publicly on your live site and risk messing it all up (with the possibility of getting in over your head).
So I made you this video to show you how you can set up a test blog on your domain (which is easier for less techy types than creating a web server on your own computer for testing).
What you see in this video may differ a bit from your hosting control panel, but the basic functions should all be there. You will essentially be creating a WordPress blog in a folder on your domain.
You can use a test blog to try out new plugins and themes. If you want to make it as much like your existing blog as possible, you can export your content from the main blog and import it into your test blog, creating a duplicate and separate copy of your blog. You can install the same themes and plugins into the test blog, too, if you want. Just install them the way you normally would on a blog (there are more technical ways of completely duplicating everything, but I’m keeping this as simple as possible).
Remember to keep your test blog upgraded with the latest versions of WordPress, themes, and plugins, just as you do with your main blog.