There are lots of example codes out there.
Basically, follow these steps, it is the same for all small PICs but note that the 16F84A does not have a USART which makes if far harder to communicate with through a serial link:
1. configure the USART registers so that transmit and receive are enabled and the baud rate (serial data speed) matches your GSM module.
2. write an ISR that reads any incoming bytes and saves them.
3. enable the RCIE bit so incoming characters are captured in the ISR.
If you are using assembly language I strongly recommend using the templates provided by Microchip to get started. The templates have all the essential ISR code in them already and comments to show where your own code goes.
Check your GSM module uses the same logic voltages as your PIC, if necessary add a level translating circuit. Don't fall into the common trap of believing you have to convert everything to RS232/V24 levels and back again! A level translator can be as simple as three resistors and one transistor.
Brian.