How To Find the Coulomb Force for Simple Point Charges

1. Identify the charge you want to find the force on; call it $Q$.
2. For another charge $q_i$, draw the force vector $F_{Q,i}$ on $Q$ due to $q_i$, taking into account whether it is attraction or repulsion. When considering $Q$ and $q_i$, ignore the presence of any other charges.
3. Find the magnitude of this force: $F_{Q,i}=kQq_i/r_i^2$, where $r_i$ is the distance between them.
4. Decompose the force into components in your chosen coordinate system.
5. Repeat for every other charge in the problem.
6. Now add the forces $\vec{F}_{Q,i}$ vectorially, i.e. add up the $\hat{i}$, $\hat{j}$ and $\hat{k}$ components. The resultant vector is the total force.

An example where this step-by-step approach will work is your pset #1 problem 1.