The force exerted by one charge q on another charge Q a distance r away is given by:
Coulomb's law: F = kqQ / r2
the constant k = 8.99 x 109 N m2 / C2.
Remember that force is a vector. When more than one charge exerts a force on another charge, the net force on that charge is the vector sum of the individual forces.
The force between charges is very similar to the gravitational force between interacting masses. The equations are very similar, and in both cases the force goes as 1 / r2.