How do I create the world I want?

Hey guys, I would really appreciate a lot if you read all this (sorry it is very long).

So I’m writing a story about this world which has humans but also other fantasy creatures such as elves, trolls and dwarves etc. I want it to be a world where the humans use guns and modern technology but there is also magic and swords etc. which is at first only used by the fantasy creatures but the humans then learn how to use it (as do the fantasy creatures with guns and technology). It needs to be set in a world which is very large but a lot of it is wasteland - I want the world to have been almost fully destroyed by something, and there are only a few civilizations and stragglers left.

So, one idea I have to create this world is that Earth has been in a long, long war, until eventually basically almost everywhere is nuked. However I don’t have much knowledge in war and things like that so I’m not sure how I could make it credible that the whole world is nuked, but I can’t think of another way to create such a wasteland in a world which was once inhabited by humans. If I used this idea, at the moment that everywhere was hit by nuclear bombs, fantasy creatures from parallel universes (or something similar) were thrust into Earth. This would somehow be caused by so much energy being released by the nuclear bombs. This moment would later be known to all as “The Merging” of all these different worlds.

The problem with this idea is that I want quite a lot of the story to be focused on fantasy creatures, so it may make more sense for it to be set in a world which was originally inhabited by the fantasy creatures and the humans then found it.

Please post your ideas for creating the world I want =) I basically have the story and characters down, I just need to find the right setting.