In order to simulate a building within TRNSYS, you have to use TRNBuild which is a EnergyPlus engine based add-on; you may need to take a look at the energyplus documentation to get a general idea on how different objects (i.e thermal zones, spaces..) are defined. Moreover, if the simulation of the building in question include HVAC and Energy management system (EMS), i would then suggest you to use either OpenStudio Application or OpenStudio SDK, who gives a complete and intuitive access to EnergyPlus along with a suite of applications including a Parametric Analysis Tool (PAT). Also all those tools are free and open source.