A maven plugin is composed of one or more mojos. And to start the creation of a maven plugin, you can use maven-archetype-mojo to create a mojo.
mvn archetype:create -DgroupId=[your project's group id] -DartifactId=[your project's artifact id] -DarchetypeArtifactId=maven-archetype-mojo
This would then create a maven project.
. |-- src | `-- main | `-- java | `-- [your project's package] | `-- MyMojo.java `-- pom.xml