Us humans are an example of Natural Selection, here's why:
For hundreds of years we have been evolving to live in our world. In the early 1700's, most humans lived to be at least 20 years old. Now, people are dying around the age of 80. We as humans adapted to the land, and learned different ways to live. In fact one of the earliest things we have done was throw spears/rocks and we were only capable of doing that because of our hand shape. We are alive today because we were (and still can) reproduce and have offspring.
Who determines who is going to survive?
The environment in which you, or the animals lives in chooses who is going to survive or not.
What would cause a human to be selected "out"?
For a human to be selected "out", it would have to be incompetent (unable to reproduce/lacks necessary skills needed to live successfully), and how your body is going to adapt to the weather. If you don't adapt, you won't survive.