First datasheet is quite useless, but second one says:
OK I found the second one and it's little better than the first. I haven't used AT commands for 20 years so unless someone else here is up to speed on them you'd better start Googling, better still ask the guys selling this to point you in the right direction.
On one sheet the chip is referred to as a GSM1, whereas the other sheet is about an SIM900, are they the same chip? The SIM900 talks about SPI, I2C etc but these atren't on the schematic. The SIM900 does however have a stack of UART_xxx pins which presumably are the handshaking lines on the first sheet.
Looking at a few of the pins they do appear to be the same.
But isn't enough to just connect RX, TX to arduino to control a serial-controlled device?
CTS/DCD/RTS/DTR etc are handshaking signals, presumably they are used or they wouldn't be there. If they are
used then some of them will need to be tied low (or maybe high).
But apart from that you're right, you should just need Tx and Rx. BUT, what are you going to say and what do you expect from the device.
You need to know
a) What are the AT commands it needs, and
b) What (if any) handshaking lines need to be dealt with.
With a chip of this complexity there should be a programmer's manual of some kind. Get onto http://sim.com
(I tried but the site was down) and see if they have a real data sheet.