Copy one site onto another site?

*I'm a novice here, so please use the most basic terminology/instructions possible*

We have a site ( that I'd like to make a copy of and post in place of this site ( After it's copied, we'll make a few modifications, but since 90% of the content will be the same, it makes more sense to copy one site over to the other, than try to manually recreate the first site.

What is the easiest way to accomplish this? They will both be hosted in the same place.

Thanks in advance!