Hello Israt! Your question is very unspecific and reads a bit like "how do I create an LCA model/can you teach me LCA modelling?" ;-)
If you haven't checked it out yet, there is a manual for openLCA and also a case study tutorial available for free on our website http://www.openlca.org/learning/
and we also offer trainings http://www.openlca.org/trainings/
Once you are familiar with the basics, I would suggest you take a look at existing processes for coal and natural gas power plants. Try to combine them under consideration of specific characteristics of combined-cycle power plants which (i guess) you will find in the literature.
I hope this helps.
Training opportunities: http://www.openlca.org/trainings/