GSM Library for SIM900

I bought a SIM900 module from SainSmart and Im trying to connect it to Arduino MEGA. The module comes with a Tx and Rx pin that obviously go to the MEGA. But MEGAs have a bunch of Rx & Tx pins. So Im trying to understand how I should interconnect these. I know this is not a SainSmart forum but I would like to understand the logic based on if I were using an Arduino GSM shield.

Attached are pictures of my module. This is the library (library here:GitHub - MarcoMartines/GSM-GPRS-GPS-Shield: GSM/GPRS & GPS Shield Library for modules using SIM900/SIM908).

I thought I had to define the Rx & Tx pins in my sketch in order for the MEGA to know where to look for Rx and Tx data, right?

Im using the sketch from that library called GSM_GPRSLibrary_Call.ino. (GSM-GPRS-GPS-Shield/GSM_GPRSLibrary_Call.ino at GSMSHIELD · MarcoMartines/GSM-GPRS-GPS-Shield · GitHub)

I arbitrarily connected the Tx & Rx from the SIM900 to the Rx & Tx 18/19 on MEGA.

Q1/ The only pin in the sketch is the pin = 1 for a digital input to use in order to send the sms. So where are the tx/rx pins defined?

Have you solved the connection between arduino and sim900? I cant find how connect it and what library use in order to send sms