Hi Alexander_Simpson.
I think the first step is to forget coding entirely to begin with, and see if you can just put together a request in your web browser that does what you want.
Check out the documentation and examples for song/search (which finds songs given parameters... things like min and max tempo, for example) and playlist/static (which generates a playlist, different every time, given a seed (an artist, song, etc) and a set of parameters like tempo, etc).
Here's a basic query to start you off that gives back 100 songs with tempo between 150 and 151 bpm: http://developer.echonest.com/api/v4/song/search?api_key=your-key-here&results=100&min_tempo=150&max_tempo=151
Once you can type the kind of query that you want into the address bar of your browser and get back results that are satisfying, then the question becomes what next? Does that do what you need, or are you trying to automate this somehow? Build a UI where you can enter things like the tempo you want and it'll generate the request for you? Or what?
-Eli