One of the most important things for you to consider when you are going to green screen is your background image. Here are a few recommendations:
When searching for "backgrounds" in Google, use the Google Search tools to search for "Large" images.
Use a website such as Upsplash that has high quality, copyright free, and perhaps HD images. Another site with some high-quality imagery is Free Wallpapers.
Consider making your own images using Keynote or Photoshop (especially if you want to include text in the background). Watch the video below to see an example of Mr. Robert Deirth's video backgrounds.
For more information about selecting your background image(s) to replace your green screen, watch this video.
Take a look at this video if you are considering replacing your background with a moving image (i.e. .mov file type).