How to find an iPhone that s already lost?

While coming home from the bus today, I realized that my iPhone was missing from my backpack and cried out in despair… My iPhone(4S) doesn't have any find my iPhone apps installed into it, nor does it have iCloud on it. It has iCloud, but I haven't set it up yet. Can someone please explain the concept of finding a phone through iCloud for me? I don't get it. I think it was stolen from my backpack during PE, but I'm not sure. There's a good chance that my phone was turned off. Help me please! Thank you!

Added (1). @Shabana Do you think if i buy a warranty right now, they will get me a new one?

Phone the network, block it thats phones gone, also see if your insured n if you r no prob they will send you a new one iut, if your not your screwd

Why not call the iPhone number. If someone answers say "Oh, Thank you! Thank you! I was in so much trouble. How can I meet you and give you your reward?" Then make a plan to retrieve it from the finder. Maybe someone just found it and is wondering what to do.