Finite element method help

I'm currently programming the finite element method to construct the stiffness matrix and RHS vector to solve poissons equation on a 2d mesh.

I've been given a vector "connectivity" which is the connectivity array and "xy_coords" which give the coordinates for each node. I've constructed the global stiffness matrix but I'm unsure how to construct the RHS vector. I'm given this:

http://img98.imageshack.us/img98/6618/feaok.jpg

I'm quite new to FEA and I have limited applied knowledge so I'm unfamiliar with centroids etc. Can someone help me with the construction / explain what the one-point quadrature formula means?