The highway from Cancun to Tulum has recently been completely repaved, making the drive quick and comfortable.  If you are renting a car and driving yourself, take Highway 307 to the south.  After passing through Puerto Morelos while on Hwy 307, watch for the highway kilometer marker 312 and Casa del Secreto. Turn left onto the Playa del Secreto road and and follow the road to the beach.  Once you near the beach, the road forks. Take the left fork. Casa del Secreto is on your right (3rd villa) with a sign in front.

To enter Mexico as a US citizen you will need either a passport or a drivers license and birth certificate.  US dollars are accepted everywhere but you might want to exchange for some pesos upon arrival to get a better rate.