Why does my SIm card not work on my iPhone?

So anytine I put any kind of SIM card in my iPhone, it says there's no service? I got like a warning that if I don't restore my iPhone I will not be able to receive or send calls? And plus I don't have iTunes so would I have to rest it or is that a completly different thing from restoring?

The phone might be blocked:/ did you get it second hand/robbed