I often like to find inspiration for scenes and dioramas from google images. So, I'll put in a key word or two (so in your case you might put "swamp" or "swamp rangers" or "swamp scene") and gather ideas from images I like. For example, looking through the swamp pictures, I like the water and presence of those fan driven boats, and I like how the greenery is looming close to everything, and I like the way the buildings often look run down. You, of course, can pick whatever you'd like. From there I would look at the pieces I have and spread things out to see how things might go together and what sort of scale they would be. I know many people like to sketch out their concept, so if drawing helps you visualize what you'd like your diorama to look like, that's great.
Then it's a matter of building up your scene, trying things out, seeing what works and what doesn't. Don't be afraid to make mistakes and learn from them. You can always take things apart and rebuild it, that's the beauty of working with Lego.
Good luck!