Status and startup for physics with the ATLAS experiment

The LHC accelerator is going to produce the first proton-proton collisions in one year from now. The ATLAS experiment at the LHC entered in a new phase, dedicated to the detector commissioning. This paper is focused on the ATLAS physics commissioning, which will proceed through four different stages. We describe here the strategy which will be followed to understand the detector and undertake the first physics measurements, from beam tests to cosmic run and from pre-collision to collision events.