I went to the golang topic of GitHub sorted by "Recently updated" and looked at repositories that do not yet have a lot of stars. Most likely they are at an early stage so it might be easier to contribute simple things, such as tests and GitHub Action configurations.

I found xhash package.

As this is the first time I am adding GitHub Actions to a project written in Go I got a bit luck as this one had a Dockerfile and a Makefile. Thus it was quite obvious how to run the tests. 

make
make test

I also looked at a more popular Golang project to see what they have and based on that I got the following config file that I saved as .github/workflows/ci.yml. Besides a small typo I got it right and now the CI runs and tests the code on 6 different platforms.

examples/xhash-ci.yml

name: CI

on:
  push:
  pull_request:

jobs:
  test:
    strategy:
      fail-fast: false
      matrix:
        runner: [ubuntu-latest, macos-latest, windows-latest]
        go: [ '1.18', '1.19' ]

    runs-on: ${{matrix.runner}}
    name: OS ${{matrix.runner}} Go ${{matrix.go}}

    steps:
    - uses: actions/checkout@v2

    - name: Install Go
      uses: actions/setup-go@v3
      with:
        go-version: ${{ matrix.go }}
        check-latest: true

    - name: Show Go Version and environment
      run: |
        go version
        go env

    - name: Install dependencies
      run: |
        make

    - name: Run tests
      run: |
        make test