Step 1: Export Gerber CAD Files From Your Layout Software
Steps to generate Gerber files in Eagle:
1. Eagle creates the necessary Gerber files with its 'Cam Processor'. To launch it, go to File -> Cam Processor.
2. You will now need to give the Cam Processor directions on which layers mean what. To simplify this, I created a CAM job file.
a. Download this file and put it in you Eagle installation directory inside the folder 'cam' (on a Mac: Applications->Eagle->cam)
b. Back in Eagle, with the Cam Processor pulled up, go to File ->Open->Job. Load in the file you just downloaded
c. Look through each tab and verify that the layers you have used are included (for example, if you used layer 200 for extra silkscreen, you will need to include it on one of the silkscreen tabs).
d. Click 'Process Job' at the bottom of the Cam Processor window
3. Go look in your Eagle design folder (typically in Documents). You should see some new files that were just created.
If this explanation doesn't do it for you, check out this Sparkfun tutorial on Eagle. About halfway down the page there is a detailed explanation of generating Gerbers and what each file represents.