In order to create gerber files, PCB Layout has to be created first. Following steps are to be carried out, in the same order:
1) Mapping components with footprints.
2) Generating a .net netlist
3) Reading the .net netlist in the Pcbnew tool.
4) Creating the PCB layout in Pcbnew.
5) Generating the Gerber files.
The order of the steps has to be followed, otherwise one will not be able to get proper results.
Locating footprints is explained in this tutorial, kindly watch it; also go through other forum questions