This may be too obvious an answer for you but here goes:
The SIM (subscriber identification module) is a small chip/card that identifies you uniquely on the network. It is accessible on the top edge of the phone between the on off button and the headphone jack.
There is a tiny hole just big enough for a paperclip to enter and you remove the SIM card by pushing gently in with a paperclip and a plastic frame containing the SIM card will pop out.
check that:
a) there is a SIM card
b) it is seated properly in the device- ie it is seated in the frame properly.
If reseating the SIM doesn't work then I would suggest fining someone you can borrow a working SIM from or go to a store and do that--this way you will know if it is the SIM or the phone.
If none of this helps, then the problem goes deeper and I'm afraid I don't have further suggestions at this point.