We describe a field-theoretic approach to calculate quantum shot noise in nanoscale conductors from first principles. Our starting point is the second-quantization field operator to calculate shot noise in terms of single quasi-particle wavefunctions obtained self-consistently within density functional theory. The approach is valid in both linear and nonlinear response and is particularly helpful in studying shot noise at the atomic-scale level. As an example we study shot noise in Si atomic wires between metal electrodes. We find that shot noise is strongly nonlinear as a function of bias and it is enhanced for very short wires due to the large contribution from the metal electrodes, while for longer wires it shows an oscillatory behavior for even and odd number of atoms with opposite trend with respect to the conductance.