PCB:NG requires the following files:
- Drill file
Once you've generated your Gerbers and drill file, you may elect to either upload them individually, or put them in a ZIP archive and do them all in one shot. Really, you probably want to go with the ZIP.
PCB:NG suggests, but does not require, Pick and Place files (also called XYRS files, and Centroid files). We also have some information on the specifics for these as well.