Start by selecting something from one of the four pulldowns. To narrow the results, select from addition pulldowns clicking the 'search' button after making changes. With each addition search term the search results will get more and more specific. Adding a keyword of your own in the text box can make it very specific.
How the songs are organized may differ from what you have in mind. One person's "sad" may be "happy" to someone else. Well maybe not happy but you get the idea. At first keep your search simple so you have more choices. Poke around a bit. Sometimes exploring is the best way to find something interesting.