Just connect RX/TX from SIM800 to RX/TX arduino (pins 0 and 1), RESET to D7.
Use a 4V external power supply to power SIM800 and don't forget to connect both GROUNDs (power supply and arduino GROUNDs).
Check this page:
http://arduinodev.com/arduino-library-for-gprshttp-communication-with-sim800