PCB:NG requires the following files:
- Gerbers
- Drill file
We have some information on the specifics for these files.
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.