Help wanted. How does gravity work?

Guest here, can someone explain to me how gravity works. I'm trying to code a quick platformer game in C++, but my program isn't working the way I wanted it to. My programming is flawless (and by that I mean it does what I want it to do), the only problem is that I'm having it do something that bastardizes physics. As you can probably guess, I've never taken a physics class before.

Gravity is a force that moves objects down at a certain rate, but how does the ground cancel out that force? Does the ground also create a force that cancels out gravity's force? 

Also, how would you go about creating a gravity program?

>>15
I'm currently not going for realistic measurements. That will be the next step after I get the unrealistic: weight, mass, momentum, and etc... variables to work.

I'm more concerned about how I'm going to go about implementing Newton's 3rd law of motion. Let's say gravity was pushing an object down at a force of 9. Does that mean that the reaction/opposite force will also exert a force of 9 on the object?

Also, I'll never take Newton's name lightly anymore. Up until now I would always say to myself, "anybody could have invented those laws if they were born at that particular time (a time when so many new theories were being presented daily)." But now I realize the brilliance behind his laws.