  This is the first paper of a series dedicated to LQG coherent states and
cosmology. The concept is based on the effective dynamics program of Loop
Quantum Cosmology, where the classical dynamics generated by the expectation
value of the Hamiltonian on semiclassical states is found to be in agreement
with the quantum evolution of such states. We ask the question of whether this
expectation value agrees with the one obtained in the full theory. The answer
is in the negative. This series of papers is dedicated to detailing the
computations that lead to that surprising result. In the current paper, we
construct the family of coherent states in LQG which represent flat ($k=0$)
Robertson-Walker spacetimes, and present the tools needed to compute
expectation values of polynomial operators in holonomy and flux on such states.
These tools will be applied to the LQG Hamiltonian operator (in Thiemann
regularization) in the second paper of the series. The third paper will present
an extension to $k\neq0$ cosmologies and a comparison with alternative
regularizations of the Hamiltonian.
