How can I configure the .gitlab-ci.yml for Maven tests

I'm newer in GitLab. I have some tests and I want to run them by CI\CD. Can you help me to configure my .yml. I should run testng.xml

This is my project: enter image description here

path to my tests is: src/test/java/tests/"files with tests"

my .yml file:

image: maven:3-jdk-7

build:
  script: mvn install -b