A bunch of people have been asking so I'll walk through the steps.
0) Install Eagle. Become familiar with the basics. Get the Eagle files for the board you want to make. Get the Eagle CAM job file for the output format you need (typically, the board manufacturer will supply a CAM file).
1) With the Eagle project open, click the CAM Processor icon in any of the toolbars (red circle) to open the CAM Processor window:
2) Click the file menu in the CAM Processor and select Open then Job...
3) Browse to the location where you saved the CAM file from the vendor and open it. This will load the vendor's preconfigured job in the CAM Processor.
The Cam Processor window allows you to specify the properties for each output file generated. The output files are used by the board manufacturer to make your PCBs. Configuration of the CAM Processor allows you to tell Eagle what information to put into a given output file. In turn, each output file tells the board house what needs to be done for a given step in the board manufacturing process. Typically, there's a set of files for the top of the board and another for the bottom. In each set, you get a file to specify the copper (where the conductive traces go), the soldermask (where the colored, translucent paint layer goes), and the silkscreen (the writing/images on the board). There is also usually a drill file that specifies location and size of holes. Generally the board outline is included in one or more of the layers - some board houses want a separate file specifying board outline though. You generally DON'T want to screw with the contents of the copper or soldermask layers, but it's sometimes desirable to alter the silkscreen layers (some CAM jobs default to having component values and names, others have just names, others have neither - it's generally OK to change this to your personal preference).
4) Click the Process Job button (red circle).
A bunch of progress bars will flash by for a few seconds. Afterwards, the folder where your Eagle project is stored will have a bunch of new files in it - one for each of the layers specified in the CAM job.
Note that the CAM job file has complete control over what is generated. You aren't necessarily required to use a specific vendor's job file to generate gerbers for ordering from them, but generally it's a good idea. Though it is also a good idea to check if they have any specifications or requirements - it's somewhat common for a vendor's job file to make a bunch of output files that they don't actually need or want to make your board, so make sure you are sending the files they actually need.
, the rest of my order will be from them..?
