Use reference photos for backgrounds. I'm not saying trace them (nothing wrong with it, a lot of famous comic book illustrators do), but definitely don't be ashamed of it. I'ts hard to imagine what buildings should look like or how the compare next to a person.
Also, write out your ENTIRE story before starting. It doesn't have to be detailed, I summarized the entire thing. But this way, if you have artist block later, you can look back and have a clear idea of what you were going for. And your story doesn't feel rushed.
Fugitive