Connecting GSM SIM900A module with Ardunio
I have a very similar board that is on the photo you posted, but with SIM900A instead of SIM900.

You can't connect the RS232 Serial Port directly to Arduino because of mismatch in voltage. The voltage on these pins is higher than Arduino's 5V. You would need a TTL to RS232 converter to hook up Arduino to those pins.

This board has also another interface to communicate. Those 2 jumpers can be removed and then you have access to 2.8V serial pins. Looking at your photo - there are 6 of them, the bottom 2 are GND, and the top 2 are RX (the one on the left) and TX (the one on the right). The bad news is you also can't directly connect Arduino to these pins. You would need a logic level shifter to shift 5V to 2.8V. I've however used a bluetooth-to-serial module from ebay that outputs 3.3V and risked connecting it to these pins. It works smoothly.