Hi Nancy,
The best way to do this is to create a staging site. This is a site on your server in a sub-directory, like yoursite.com/staging
Then you can build the site in that sub-directory while leaving the live site alone.
Then when it’s ready, you can move the staging site to the live directory using a plugin like Duplicator.
Some web hosting companies have staging options that make the process much easier – might be worth asking yours if it’s possible 🙂