A couple things that may be helpful: * Keep functionality in mind. Fun should always trump realism, but you can add a lot to your maps by spending some time thinking about how someone would live/work/eat/sleep/etc. in them. Imagine walking into the room, using it for its intended purpose, and moving around in it a bit. Rearrange the furniture until it feels comfortable. * Use examples from real life for inspiration. Given that you're specifically asking about a modern setting, this will be easier than it would be for other settings. Look around your house, your work, the stores you visit, etc. for rooms like the one you want to make. Mute an episode of a TV show you've already seen and watch the background instead of the action. * Pay some attention to tactics, if applicable. If you're going to have a shootout in your map, place some furniture that can be used as cover within the characters' movement distance of each other. If you want close-quarters fighting, shrink rooms and place tall furniture to break sight lines. If you want people to sneak around, place cover and try to ensure that there are muliple paths characters can take (to enhance the cat-and-mouse between the sneak and the security patrol).
