arduino uno with gsm modem sim 900a

Hi guys,

I am using arduino Uno with GSM modem sim900a, its modem, so i am using GND, VCC pin from modem to connect arduino's 5v and GND, RXD and TXD from GSM modem pin connect to Arduino's RX0 and TX1. but while uploading sketch, it gives me stk500_getsync(). so i thought, its problem for serial connect. then i changed RX and TX on arduino to 5(RX) and 6(TX) pin, to using these pins I am using softwareserial library. but again i couldn't transfer any data. serial terminal shows nothing. Pls can you help how to use GSM modem(not shield, have to wire up....), even I tried to connect putty.exe to my gsm modem, couldn't receive anything.

Guys give me ideas? :)))