To get started using the action, just make sure to have a Procfile or a Dockerfile in your project and then create a folder called .github and inside it, create another folder called workflows. Finally inside the workflows folder, create a file called main.yml with the following contents:

.github/workflows/main.yml

name: Deploy

on:
  push:
    branches:
      - main

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - name: Deploy to Heroku
        uses: abunch/heroku-deploy@v1
        with:
          app-name: "your-app-name-on-heroku"

You can now push your project to GitHub and it will be automatically deployed to Heroku henceforth.

You learn more about GitHub Secrets here and GitHub Actions here