Check with the front desk of your hotel. They will guide you to tour companies who are licensed, and will book the tours for you if you want.
In addition, just exercise the same caution that you would anywhere. Don't walk around late at night with a wad of cash in your pocket, and don't be naive enough to let anyone know where you are staying. Most of the crime on the island is petty thievery, and usually because someone knows what you have. Keep your cash, Iphones and computers in your room. If you are out late, take a taxi to your hotel.
And there is always the falling off your barstool!!