Hi, Anastasia

First of all, you should design the thermal circuit of the plant (steam generator, steam turbine, condenser, regenerative heat exchangers, pumps). After that, you should draw the expansion line in the turbine, finding the steam enthalpies from inlet to exhaust.

The following step is to compute by mass and energy balances the steam rate flows that leave the turbine to the heat exchangers, in order to obtain the mass flow rates of the steam that expands. The internal power represents the sum of the flow rates x enthalpy drops of each turbine section. Multiply this with mechanical and electric generator efficiency and you have the electric pover produced by the steam turbine. 

http://www.steamshed.com/pdf/038SteamTurbinesLargePowerApps.pdf

http://www.geothermalcommunities.eu/assets/elearning/7.29.steam-turbines-for-geothermal-power-plants_en.pdf

http://lab.fs.uni-lj.si/kes/erasmus/Steam%20Turbines%20for%20Modern%20Fossil%20Fuel%20Power%20Plants.pdf

https://www.theseus.fi/bitstream/handle/10024/86896/Operation%20and%20maintenance%20schedule%20of%20a%20steam%20turbine%20plant%20and%20flow%20diagram.pdf?sequence=1