A zero-dimensional code is described for modelling the plasma formation and preheating in a Tokamak, by means of extraordinary waves in the electron-cyclotron frequency range. Numerical results are presented, which give the increase in time of the electron density and temperature, together with the power balance in the steady-state plasma Extrapolation of these results, shows that the RF plasma in larger size Tokamaks may reach temperatures larger than the impurity radiation barriers.