Although Newton's theory of gravity has been overthrown by Einstein's Relativity, It still holds true that one can make accurate predictions about the motion of objects using Newtonian Mechanics. For instance, things such as the acceleration of gravity (on earth) is derived from Newtonian Gravity, and is very accurate. It would probably be a very good Idea to look at the theory before moving on to more advance things. (especially if you are like me and your math skills reach their limits past Newtonian Mechanics.) I like the equations from Newton's gravity because you can understand them with just basic algebra, geometry and science behind you pretty easily. Furthermore these equations have been used to understand one of the most fundamental forces for hundreds of years, and is a very useful thing to know regardless.
Step 1: The Equation
The equation that is used to find the gravitational force between two objects (typically one very massive and one less so) is pretty simple, and some what elegant to me. F= (mMG) /D2 Let's begin to dissect this. F stands for Force, or more specifically the force of gravity. m stands for one of the masses involved in the gravitational interactions typically the less massive one. M stands for the other object involved in the gravitational interaction, typically the more massive one. D would be the distance between the objects. G is a particularly interesting part of this equation (equally essential). You may have noticed that it seems small things are not attracted to small things. Certainly you do not have less massive objects circling you right now. This is where the variable G or the Gravitational Constant comes into play. if you were to exclude it from the equation you would notice that any two things with mass would attract each other with a good amount of force even from macro distances. Obviously this doesn't happen. the attraction between two objects that are not as massive as planets is not even enough to really notice (none the less is there, but is just very tiny).
Go to the next step for the gravitational constant.