If you have a WordPress site and want to be able to create a test site to make changes to before uploading to the live site then it may not be as easy as you think. Ideally you should be able to download a copy of your database and your files then reload them to the sub domain or new domain, but you also need to remember to change any mentions of the old URL in order to get the site to work correctly.
You may be able to do a find and replace in the database in order to get this done or you may have to manually go through and change it. You will also need to consider the HT access file to ensure that it does not try and redirect the test site to the live one. Even if you do all this correctly you can still end up with corruptions and find that the styling is missing from the site or from the admin area. This can be a nightmare and often results in you needing to copy all the files again and re upload them to make sure that it has been fixed. Some hosting companies will be able to provide this service for you but will often charge a fee to do so.