What is the easiest way to add data to an EXIF file? For example if I wanted to add a copyright to my images can that be easily added to the EXIF data?
The easiest way to add exif data is to do so in camera. Not all camera models allow for setting the copyright in body however, so the next best way is during image import. I use lightroom as the hub for my digital workflow, and it allows me to specify any and all exif information I want. You'll want to check with your image import utility and verify it has exif support.
There's a comprehensive list at http://exif.photofb.com/how-to-edit-exif-info.php
Some cameras, support setting an owner name on the camera, which will end up in the EXIF metadata on the picture.
There are thousands of programs for adding extra keywords and other information, and you'll find people rave about their favourite. Personally, I use Microsoft Pro Photo Tools which happens to be free, and allows me to add information as to where the photo was taken ("geotagging") as well.
Photoshop will allow you to add your copyright information etc to the EXIF, it also allows you to save templates of your common information for easy addition to new images.