Name your file
The image will upload. Enter a unique name for the file, something you think someone might search for to find the file. This name will also become the name of the page. Then enter a description.
You should add categories to your image, too. Categories will put your uploaded media into a general catch-all page where people can find it. For example, "Category:Bonsai" collects all the photos of bonsai trees that were uploaded (and tagged) on Commons.