So how big is the force of gravity acting on you? Well how much do you weigh? The answers are the same! To see this step up onto a scale. Then we have the following diagram:
The red arrows indicate the forces acting on you. They
are the force of gravity, and the force the
scale exerts on you. The force the scale exerts on
you is an example of a ``normal force''.
The blue
arrows indicate forces you exert on everything else.
So at this point it is always a good idea to consider only
the forces that act on one object at a time. So lets consider the
red arrows first.
We want the sum
of these two forces to be zero because you are not
accelerating. Therefore the force of gravity 
plus
the force of the scale on you 
to be zero.
This means they must have the same magnitude. So 
.
Now what about the force you exert on the scale, 
? By the third
law, that has the same magnitude as 
. So the scale registers
a weight of mg.