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Before you know where you should put what things within the project, I suggest a brief reading on Getting Started by Maven. This will help you understand various details and conventions used in a Maven project, so you don’t understand the dynamics of the tool, and not that "it should be like this because someone just told me".
Your question is related to the standard package structure used by Maven, and for what it serves each of them, this convention can be found in the Standard Directory Layout of documentation:
In your case, application-related packages and source code should be placed in src/main/java
and src/main/resources
.