By "ID Scanned" do you mean a scanner such as a bar code, magnetic strip or RF tag scanner? Almost always, these can run in "Keyboard emulation mode". If your scanner does, you do not need a plug in for this, you can use script triggers instead.
That said, start with the help entry In FileMaker Pro Advanced: (this is just the opening paragraph.)
Creating plug-ins (FileMaker Pro Advanced) If you are a C or C++ programmer and familiar with calculations in FileMaker Pro and FileMaker Pro Advanced, you can create external function plug-ins that extend the feature set of the applications. The plug-ins can take advantage of recursion and looping or hook into other programming interfaces. Users can enable your plug-ins in FileMaker Pro, FileMaker Pro Advanced, FileMaker Server, and FileMaker Server Advanced and use your external functions in their calculation fields and scripts. You can use FileMaker Server to ensure that FileMaker Pro clients always have the most current plug-in software installed on their computers. See the FileMaker Server Guide to Updating Plug-Ins, available at www.filemaker.com/documentation. To see an example plug-in, go to www.filemaker.com/support/technologies.
Its a OCR scanner. I want it to scan Drivers Licence Picture (into a container field) and personal Information and update my customer profile.