I have a maven project and when built it is an EAR. I need create a folder inside EAR structure so it will be "APP-INF/classes" and place a file inside. What plugin or tag in ear plugin should I use? Thank you in advance.
You could use the maven-assembly-plugin. Write your own descriptor to package the .ear just as you want.
As a side note, The APP-INF/classes/ folder is a proprietary feature (from Weblogic I believe). If you want to be cross-platform and compliant with the Java EE spec, You should package all the classes/ contents in a .jar. See this and Section 8.2.1 of the Java EE 6 Spec