Just about every day I have people who are looking for proofreading marks end up at this site. I feel bad, so I decided to put together some information to help.
First of all, not all publications use the same proofreading marks, but there are some common ones.
If you are working at a newspaper or other organization that uses AP style, I would recommend buying an AP Stylebook. A full list of proofreading marks is in the back of the book.
Also, there is the The Chicago Manual of Style , which uses many similar proofreading marks (plus more).
Here are some examples of how you might mark a text. Also, some publications mark in the text while others mark in the margins. Whatever way you want to do it, it does make it easier if everyone in your organization is using the same proofreading marks and marking them in the same fashion.