The image will upload. You'll see a thumbnail (sometimes the thumbnail doesn't load — don't worry, it's still there!). You'll want to enter a unique name for the file, which will also become the name of the page. Choose something you think someone might search for to find the file. Then enter a details 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.