How to interface Reed Switch sensor with 8051 Microcontroller?

Hi

I want to build a system contain sensor ,Microcontroller and GSM module. I'll use SIM900 GSM module. The main idea of this project if Reed switch sensor connected I want the GSM module send an alarm message to a mobile.

Can you help how I can do this?? some steps and hints.
For GSM module I read about AT commands and I knew I must desire if I want it receiver or sender.

Any suggestions it's will be better.
Thank you