Realistically, as a tourist, you only need to think about using the CUC. Anything you pay for, so accommodation, hotels, taxis, meals in restaurants etc, will all be CUC. The only time you would consider paying in CUP is if you are adventurous and use local transportation (buses aka ‘guaguas’). Or perhaps buying small bits of food in a market or on the street.
You might be confused in the beginning about when to use which. Ideally you will have understanding of Spanish to be able to get your head around it all, too. For example: you’re in a market and you’re buying some bananas. The vendor says “20 pesos”. You can be pretty much certain that, unless these bananas were made of solid gold, he’s not asking for 20CUC (aka 20USD) for a couple of bananas. He’s wanting 20CUP, which is a little less than 1USD. Got that?