We present a new particle code for modelling the evolution of galaxies. The code is based on a multi-phase description for the interstellar medium (ISM). We included star formation (SF), stellar feedback by massive stars and planetary nebulae, phase transitions and interactions between gas clouds and ambient diffuse gas, namely condensation, evaporation, drag and energy dissipation. The latter is realised by radiative cooling and inelastic cloud-cloud collisions. We present new schemes for SF and stellar feedback. They include a consistent calculation of the star formation efficiency (SFE) based on ISM properties as well as a detailed redistribution of the feedback energy into the different ISM phases. As a first test example we show a model of the evolution of a present day Milky-Way-type galaxy. Though the model exhibits a quasi-stationary behaviour in global properties like mass fractions or surface densities, the evolution of the ISM is locally strongly variable depending on the local SF and stellar feedback. We start only with two distinct phases, but a three-phase ISM is formed soon consisting of cold molecular clouds, a warm gas disk and a hot gaseous halo. Hot gas is also found in bubbles in the disk accompanied by type II supernovae explosions. The star formation rate (SFR) is ~1.6 M_sun/year on average decreasing slowly with time due to gas consumption. In order to maintain a constant SFR gas replenishment, e.g. by infall, of the order 1 M_sun/year is required. Our model is in fair agreement with Kennicutts (1998) SF law including the cut-off at ~10 M_sun/pc^2. Models with a constant SFE, i.e. no feedback on the SF, fail to reproduce Kennicutts law.
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