Ignore what vsathish said, there is only one hardware serial port on the Uno, and that uses (digital) pins 0 & 1. You do need SoftwareSerial to use pins 2 & 3.
To start with I would check the basic functionality by using SerialRelay ( from here amongst other places), set SoftwareSerial to use pins 2 & 3 and start with the standard convention of connecting Tx -> Tx, Rx -> Rx and ground -> ground. I assume you are using an external power supply to the SIM900?
Then use Serial Monitor to get it to respond to the AT command. Once you have it working you can integrate the relevant parts into your own scripts.
If you have any further problems please ask, and unlike vsathish, continue to use code tags.