Help! Text messaging not working on iphone 4s

I started using my new iphone 4s yesterday but can not send any text messages to my contacts. I mannaged to get an iMessage to another iphone user but normal text messages to anyone who is not using iMessage won't go through. I have tried switching the phone off, taking out the sim, and a few other things in settings, but so far nothing has worked. Should the numbers in my contacts all include country code perhaps? I am in Australia.

Please help.