how to interface a sim 300 gsm modem to a ubuntu desktop
 

My desktop doesn't detect my gsm modem (sim 300). I don't know a great of things about it (manufacturers and other details) as it was the only modem our college had and looks like a chinese model but worked well in windows hyperterminal.
I need to use it in my desktop which has ubuntu 12.04 .
I connect it to my desktop using a RS-232 cable.
Guys any help.

You could try using minicom to connect with. Or you could use wvdial to make a 'standard' Internet connection, although you will need to create some chat scripts for that.
Read this page page that discusses dial-up modems for Ubuntu.
(note the comment on that page about having the modem connected at boot-up to get it to be recognised)