First, you must know that the statement "An object at rest tends to stay at rest. An object in motion tends to stay in motion unless acted upon by an outside force." is true, because it is the first Law of Newton or inercy law.
what outside force acts on a baseball when it is thrown straight like a pitcher pitching a ball?
After the ball leaves the pitcher's hand, it is subject only to the gravitational attraction of the Earth. That is why the pitcher has to give the appropiate impulse in order to the ball reaches the point that he and the catcher want.
What about if you threw it straight into the air?
It is the same thing. The only force would be the gravitational attraction of the Earth.
What about if you threw the baseball in outer space. Would there be any forces to slow that down?
In outer space, at the beginning the baseball would be very far from of other massive objects to feel their gravitational field, so there would not be any forces to slow it down. Although eventulally, after many light-years, it would enter the gravitational field of a galaxy or other massive body and it would attract it.
