As you become experienced in using Audacity, you will discover many ways to edit and improve your audio files. You will also discover an endless series of online tutorials to show you how to do exactly what you need to learn. The skills that you should begin with include using these editing tools:

Please watch this video on creating an audio file using multiple tracks.  If you have access to audacity on your PC you can follow along and try it out for yourself.

Sound effects

Sound effects are any sounds that are created artificially or added after the main audio is recorded. Sound effects can provide ambient sounds when the main recording is unable to capture useful sounds.

The creation of artificial sounds that represent something real in an audio presentation is similar to the Foley art of the movie industry. For example, creating the sound of horse's hooves with coconut shells or crinkling a plastic bag to suggest tires on a gravel driveway use sound props to simulate real-life sounds.

With this knowledge add sound to your audio in Audacity