Hi Scarlett. As everyone else is saying, there are so many fantastic places to see and many different ways of seeing them! Also, it would depend on whether you want to see some exciting cities or whether you'd like to see some of the amazing natural places (or a bit of both)! We have been to USA several times and enjoyed every minute of our trips-we hire a car and go as we please. Parts of Utah are fabulous-Bryce canyon, Arches national Park and Canyonlands north NP too. The Rocky mountains too are so very beautiful especially in the fall when the leaves change colour. Yellowstone NP is unmissable in my opinion-it's unlike anywhere else on earth with the geothermal activity and the wildlife. But then, Las vegas is worth a visit as well. Oh so many things to see over there!! This year, we are starting at San Fransisco, driving up the coast as far as Astoria then heading back down through Oregon and California, via Yosemite, Tahoe and Crater Lake then looping back to San fransisco.
There's loads of maps available on say, Amazon and some very good books (especially the Frommer guides) to help you choose where to go and of course the internet is great for looking at images of places and Trip advisor is worth a look.
I am sure that wherever you decide to go, you will love it Scarlett!!