geologic:
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).
Thank you, geologic. And can you help me with this changes in SIM800.h?
// change this to the serial UART which SIM800 is attached to
#define SIM_SERIAL Serial1
What must I write in place of "Serial1", or I don't?
Thanks.