We recommend flying into Cancun. Tulum is approximately 1.5 hr drive south of Cancun airport. The drive to Tulum is on a well-paved, four-lane highway. You can either rent a car or get a Taxi to your hotel. Every time we've gone, we've rented a car, largely because it wasn't very expensive.
The Tulum beach road is easy to get around by bike or on foot. Most hotels rent bikes by the day. Taxis are plentiful and safe.
Credit Cards are accepted at established restaurants and hotels. US Dollars are also accepted in most places, including taxis.