If you new to WordPress and blogging then your possibly having fun playing with different themes, customising you blog to get the look and feel you want.
Playing with your live site and affecting your visitors is probably not the best idea. You need to be able test without impacting on your visitors.
A quick google round the internet soon found the answer, a plug-in that enables the admin user to test a different theme, then what the other users are seeing.
Check it out you might find it useful.