Flocking, the collective coherent motion of a large number of agents or organisms is one of the most ubiquitous phenomena in nature. It ranges from school of fishes and flocks of birds to gliding myxobacteria and elongated granular matter driven by an external force. Here we concentrate on the universal features of this phenomena beyond the specific details of each situation. To this aim we study a class of minimal microscopic model in which point-like self-propelled agents communicate by local interaction rules only. It is shown that, upon changing the density of the flocking or, equivalently, the amount of environmental noise, a transition from disordered to collective motion takes place. Analysis is carried on in both two and three spatial dimension and the peculiar out of equilibrium properties of the ordered collective phase are investigated via numerical simulations.