Learn some (basic) Spanish:
Learning some basic Spanish words will make your life easier. Not everybody speaks English, and being able to communicate basic things with everyone is useful. Moreover, Cubans have their own slang that is different from Spanish in most other places. It might be useful to familiarise yourself with it before you go there.
After arriving in Cuba: Get your head around the confusion of Cuban money.
Getting around Cuba:
Cuba can be a lot more expensive than people expect, and a big part of the cost is the cost of getting around in the country. Finding out how transportation in Cuba works can be extremely useful if you want to make sure you aren’t scammed by taxi drivers.
Internet in Cuba: it's not what you are used to.