A gitignore file specifies intentionally untracked files that Git should ignore. Files already tracked by Git are not affected.
You can find more information about how to edit this file in gitignore Manual Page. In general Git GUI editors have tools to help you to edit it.
We provided a recommended .gitignore file to use in your projects, it is mainly prepared for Visual Studio Projects, but it also have entries for Jekyll, XCode, Ruby, etc. You can use it as reference, take some parts or the complete file. See it at our GitHub repository.
Some text extracted from http://git-scm.com/docs/gitignore (CC BY SA 3.0)