Use "there's ___" when you're describing a situation that's happening now:
There's someone at the door.
There's some food in the kitchen, if you're hungry.
Or something that's planned for the near future:
There's a game coming on at four.
It's also common to describe what something is doing using "there's":
There's a woman staring at you over there.
There's a humming noise coming from the refrigerator.
There's some cake sitting on the counter.