IPhone 4 still on No Service?

I bought my IPhone4 exactly a week ago and i have inserted the new sim card and set up the phone through IPhone. I have connected to my Wi-Fi but in the top left hand side it still says 'No Service'. Does anyone know how to solve this?
All answers appreciated.

Yes. Is the phone branded to a certain carrier or unlocked for SIM. If it's unlocked, call the carrier and ask for assistance. If it's not unlocked, and you're using another SIM from a different company, you have to get it unlocked.

You didn't provide enough detail to me be more helpful.