The US dollar is the official currency used in the USA and is the most traded currencies in the world. A number of countries call the US dollar their official currency, so you should have no issue obtaining US dollars before your trip.
Alternatively, you can exchange your currency in the USA, at the airport or in major cities. ATMs that accept major debit and credit cards are abundant throughout the US, so it is not necessary to only carry cash.
Due to international political uncertainties, such as Brexit, it is tough to predict what the exchange rate will be in the future. At the time of writing, the exchange rate is quite weak, fluctuating around 1 GBP = 1.3 USD. This means that items in the US will be more expensive now. Before Brexit, the rate was around 1 GBP = 1.6 USD.
Tipping is standard in the USA, so keep some loose dollar bills to give to hotel employees, taxi drivers and restaurant workers, to name a few.