Hi Joanio,
There are 2 currencies in Cuba.
There is the ordinary Cuban Peso which is what the ordinary Cubans are paid in and which they use for their normal day to day transactions. There are "Peso shops" that cater especially for the local population.
Then there is hard currency which is what you will be using. This used to be the US$ until this was abolished some years ago and replaced with the Cuban Convertable Peso. This has the same value as the US$ Known as the Cuban Convertable this is abbreviated to the "Cuc" .
Going back in time there were always shortages and there is a currency exchange at the airport but you are just as well catered for by skipping the queue there as all hotels these days can change money for you.
Assuming you are UK based then sterling is easily changed and, as Mildred says, the US$ is no longer accepted there.
You should have no trouble changing money in Havana, Guadalavaca, Santiago de Cuba & Trinidad. The other places are a bit of the beaten track & whilst they will have cash then there could be a problem if they get a rush on so I suggest you make sure you have enough ahead of those days.
Mildred has highlighted issues with cards and although the times are changing then any card with a USA connection - i.e. American Express - was useless there. I always took cash for day to day expenses & used the card (Visa) only where needed - usually only in recognised tobacco shops before I gave up.....
How much to take?? Depends on how much rum you are drinking and how many cigars you will be smoking!! There are restaurants for all budgets and the range of souvenirs can be a bit limited. Last time I was there I allowed a budget of (I think) about £40 a day and spent nowhere near that amount...
Ian