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.